FIELD GUIDE TO INSECTS OF BRITAIN AND NORTHERN EUROPE E-Book

Bob Gibbons

• Herausgeber: Crowood
• Kategorie: Lebensstil
• Sprache: Englisch

Seitenzahl: 405

Veröffentlichungsjahr: 2011

Beschreibung

Written by well-known naturalists and photographers, this guide will enable the easy identification of around 1,000 of the more common insects found in the region. The carefully chosen selection represents all insect groups, with a bias towards the more prominent species, so that all the butterflies, grasshoppers, crickets, damselflies and dragonflies occurring in Britain have been included. Over 700 colour photographs show the species in their natural habitats, and around 100 line-drawings clearly show important features, enabling accurate identification.

Leseprobe

Field Guide to

INSECTS

of Britain and Northern Europe

Bob Gibbons

The Crowood Press

First published in 1995 by The Crowood Press Ltd, Ramsbury, Marlborough, Wiltshire, SN8 2HR

www.crowood.com

This e-book edition first published in 2011

© Text, Bob Gibbons 1995

All rights reserved. This e-book is copyright material and must not be copied, reproduced, transferred, distributed, leased, licensed or publicly performed or used in any way except as specifically permitted in writing by the publishers, as allowed under the terms and conditions under which it was purchased or as strictly permitted by applicable copyright law. Any unauthorised distribution or use of this text may be a direct infringement of the author’s and publisher’s rights, and those responsible may be liable in law accordingly.

ISBN 978 1 84797 369 6

Artwork by Christine Hart-Davies

Photographic Acknowledgements

Photographs supplied by Natural Image. All taken by Bob Gibbons and Peter Wilson except for those below. Each photograph is referenced by a page number followed by a number indicating its position on the page working from left to right and top to bottom, David Element 55.6, 89.4, 93.4, 95.2, 123.1, 135.2, 157.1, 159.3, 161.2, 163.3, 69.4, 181.1, 185.1, 189.3, 191.1, 191.2, 191.4, 193.4, 195.4, 199.3, 207.3, 215.1, 219.2, 219.5, 223.6, 225.3, 229.3, 229.4, 231.4, 233.1, 243.4, 245.3, 255.1, 255.2, 255.3, 257.1, 257.4, 261.1, 263.1, 277.4, 295.4, 299.5, 301.2; Robin Fletcher 159.6, 259.2; Alec S Harmer 123.3, 125.1, 125.2, 125.5, 127.1, 129.5, 131.4, 131.5, 131.6, 135.1, 135.5, 135.6, 139.5, 141.7, 141.8, 45.3, 149.4, 155.6, 157.2, 169.2, 171.2, 171.3, 173.3, 283.3; Tom Leach 57.4; RJ Orr 159.4, 271.2; PR Perfect 39.2, 105.1, 159.2, 167.2, 169.3, 171.5, 177.1, 179.4, 185.4, 195.1, 277.2; Michael Woods 153.3, 261.4.

Contents

How to Use this Book

What is an Insect?

The Structure of Insects

How an Insect Works

The Life History of Insects

The Ecology of Insects

The Conservation of Insects

Finding Insects

Photographing Insects

The Classification of Insects

An Illustrated Key to the Main Groups of Adult Insects

3-Pronged Bristletails, Order Thysanura

2-Pronged Bristletails, Order Diplura

Springtails, Order Collembola

Mayflies, Order Ephemeroptera

Dragonflies and Damselflies, Order Odonata

Stoneflies, Order Plecoptera

Grasshoppers and Crickets, Order Orthoptera

Cockroaches and Mantids, Order Dictyoptera

Stick Insects, Order Phasmida

Earwigs, Order Dermaptera

Psocids or Book-lice, Order Psocoptera

True Bugs, Order Hemiptera

Thrips, Order Thysanoptera

Fleas, Order Siphonaptera

Ant-lions, Lacewings and relatives, Order Neuroptera

Scorpion Flies, Order Mecoptera

Butterflies and Moths, Order Lepidoptera

Caddis-flies, Order Trichoptera

True Flies, Order Diptera

Ants, Bees, Wasps, Sawflies and their Relatives, Order Hymenoptera

Beetles, Order Coleoptera

Stylopids, Order Strepsiptera

Plant Galls

Bibliography

Useful Addresses

Index

A comman ichneumon wasp, the Yellow Ophion.

How to Use this Book

This book is intended as an introduction to the marvellous range of insects to be found in Britain and adjacent parts of north Europe (see map for area of coverage). No single portable volume can possibly cover the full range of insects that occur within this area, and this book is highly selective in the insects that it includes. Although the whole range of insect orders is covered, within this framework I have selected examples that have one or more of the following features:

• They are readily noticed by the average naturalist, either singly or en masse. This includes species that may be noticeable by some aspect other than simply their visible adult stage. For example, some larvae are especially noticeable (even though their adult phase may be drab or inconspicuous); some insect products are especially noticeable, such as the froth, known as ‘cuckoo spit’ produced by froghopper nymphs; and occasionally insects may be more noticeable by their sound than by their appearance, such as house crickets. As far as possible, it is the conspicuous feature that has been illustrated, since this is what people notice most.

• They are reasonably frequent and widespread. There are a few exceptions to this, where particularly distinctive insects are involved; or within the key groups of Odonata (dragonflies and damselflies), butterflies, and Orthoptera (grasshoppers and crickets), where we have aimed to include all UK species, and most NW European species, whether they are common or rare.

• They are identifiable in the field. In practice, this is not as simple as it sounds, since many apparently distinctive species have a number of close relatives that differ only in minor characteristics. In these cases, it is the group of species that is distinctive and identifiable, and this is usually made clear in the text. Apart from the main groups of larger insects, most groups of insects require specialized texts and detailed study for their certain identification. Some of the more appropriate specialized guides are listed in the Bibliography.

The black line delineates the main area covered by this book.

IDENTIFYING INSECTS

When using the book to try to identify an insect, ideally you should have the book there at the time; it is designed to be portable, and used in the field. It is surprisingly easy to see an insect that appears to be highly distinctive at the time and to find, on later examination of illustrations, that there are several similar-looking species. It can then be very difficult to recall which key features ‘your’ insect possessed!

If you already know which order the insect belongs to, it is a quick process to flick through the pictures to see if there is anything like it. If so, check the timing and distribution to see if its occurrence is likely, then read the ‘similar species’ to see if anything fits better. If you are uncertain where the insect fits into the scheme of things, you can use the illustrated key to the main groups of adult insects. For this, you need to be able to see the details clearly; it can be helpful to catch the insect and examine it in a clear container, releasing it afterwards. Alternatively, you can scan through the photographs in the whole book, leaving aside groups to which the insect obviously does not belong. For example, if you had a stonefly, it would be readily obvious that it was not a butterfly, dragonfly, grasshopper or cricket, even though you had no idea what it actually was. Once narrowed down, the procedure is the same as above.

LAYOUT OF SPECIES DESCRIPTIONS

The layout of each species description follows a roughly constant pattern, though this is necessarily altered at times, according to the type of insect.

• If the whole description is preceded by the symbol , this denotes that the insect does not normally occur in the UK. This speeds up the process of checking through possibilities if you are working within the UK.

• This is followed by the English name, where there is one. Many insects are not well enough known to have an English name, and in such cases the description starts with the scientific name, in italics. In a few instances, where the name has been changed recently and the old one was familiar, alternative scientific names have been given.

• Next comes a description of the insect, beginning with a general indication of size, shape, colour and key anatomical features, particularly mentioning any variations from the illustrated type and highlighting features which need close examination.

• The section beginning Habitat describes the habitats in which the insect commonly occurs, indicating any differences between countries, and especially differences between the UK and mainland Europe, if appropriate. (It is surprising how often such differences exist.) This section can only be a guide, as many insects occur in too wide a spectrum of habitats to mention them all, and others – such as larger dragonflies and butterflies – can range widely through almost any habitat, if only in passing.

• The part of the description beginning Status and distribution indicates the rough distribution of the insect, and its relative abundance. Where appropriate, the UK distribution is described separately. The terms used, in order of decreasing abundance, are: abundant, common, frequent, local and rare, qualified as necessary, though they should only be taken as a guide, and there can be enormous annual variations with some insects. There are also many species for which this information is simply not fully available.

• The abbreviation Season indicates the period when the insect is most likely to be seen. Unless otherwise qualified, it refers to active adults. There is a good deal of potential variation within this, according to geographical location, the weather in a particular year, and the habits of individual species. Generally species appear earlier and survive later in warmer places: in the UK, coastal south-western areas are especially mild, and on the Continent, the W coast of France, and southern areas in general, are the warmest. A mild frost-free autumn may extend the flight period of many summer insects well into November. Some insects, especially aquatic ones, may remain adult all year, but they become less active, or totally inactive, in cold weather; in these cases, only the period during which they are most likely to be seen are given.

• A subsection headed ‘Similar species’ may follow the general description. This describes closely related or very similar species that are not usually illustrated. In many cases there are large numbers of similar species, and it is not possible to describe them all. Occasionally, where one insect may be confused with another that is described elsewhere in the book, this section may draw attention to this possible source of confusion.

ORDER OF SPECIES

The order of species follows the generally accepted order of families, progressing from the most primitive to the most advanced. Species are all grouped taxonomically, with related species placed together. There is one exception to this: all the galls have been collected together at the end of the book in a special section. Galls are a fascinating study in themselves, and the larger ones readily attract attention. However, it is not possible to classify the insect that caused the gall simply by looking at the gall, and the insects themselves are rarely seen, so it is more useful to group all these insects together. Most such insects come from the orders Hymenoptera or Diptera, though a few other groups are involved, and there are also a number of non-insect gall-formers. Common examples of non-insect galls, such as those caused by mites, are included. Where the gall-forming insect is also likely to be noticed in its adult stage, it appears under the normal taxonomic grouping as well.

What is an Insect!

The term ‘insect’ refers to a vast group of animals, belonging to over 30 different orders, with a wide variation in structure and behaviour, so it is difficult to describe common characteristics which apply to all of them. The insects are one group of the huge biological tribe (phylum) known as the arthropods, which also includes spiders, crustaceans, millipedes and many other groups, whose primary characteristics include a hard external shell or skeleton, and soft flexible joints at appropriate places which allow the animals to move. These are collectively known generally as ‘invertebrates’ (i.e. animals that have no backbone). Insects are most likely to be confused with other arthropods such as woodlice, spiders or centipedes. Their main distinguishing characteristics are:

• Insects have six legs, in three pairs. Many insects have one or more pairs of legs missing, modified or reduced, but virtually all insects have six legs at some point in their life-cycle.

• Most insects have wings at some point in their life-cycle. If an invertebrate has wings, it must be an insect. However, a small group of insects (the Apterygota, see pp.38–41) never have wings, and there are species scattered through the insect orders that have lost their wings through specialization; e.g. the fleas.

• Insect bodies are divided into three sections: head, thorax and abdomen. The head usually bears one pair of antennae (though these may be very small); the thorax bears the legs and wings, if present; the abdomen never bears legs, though it may have outgrowths associated with mating or other processes.

Immature stages of insects, such as caterpillars, are often much more difficult to categorize, and smaller examples could easily be mistaken for some non-insect invertebrates. There is no guaranteed way of identifying an immature stage as an insect, though many larvae and most nymphs retain the characteristic three pairs of legs.

The Structure of Insects

Nowadays, with good reason, most of the interest in insects centres on their ecology, behaviour and economic or conservation significance. However, in order to identify them, it may be necessary to understand their general anatomy, and their role in nature can be better understood if some aspects of their structure and biology are known.

As already described, the insect body is divided into three main parts: head, thorax and abdomen. There are basically 20 segments in an insect body, with six in the head, three in the thorax, and 11 in the abdomen. By no means all of these are usually distinguishable, as they have become fused together without visible joins. The segments are protected by hard plates, known as sclerites, composed mainly of chitin, which protect the internal contents. Between these plates, there are flexible joints, which may or may not correspond to the divisions between segments.

THE HEAD

The head varies enormously in shape from one insect to another. The six structural segments that form it are welded together to form a tough capsule, and usually the segmental divisions are not visible. The head bears the antennae or ‘feelers’, the mouthparts (which vary widely in structure), and the eyes. Parts of the head are given specific names, as shown in the accompanying diagram of a grasshopper head.

Antennae types

The antennae are organs of smell and touch, and virtually all insects possess them, even in their younger stages. There is one pair only, and they vary enormously in structure from being virtually absent to being much longer than the body. Their size and structure relates to their function, and a number of different types can be recognized. For example, male moths use their antennae to detect the presence of female moths, and they are receptive to just a few molecules of scent produced by a female that could be a considerable distance away. This ultrasensitive detection of scent is made possible by the finely divided nature of the antennae, producing a much greater surface area for molecules to land on. By contrast dragonflies, which have highly developed eyes which they use for finding prey, have very reduced antennae. Some common types of antennae are shown in the diagram. The number of segments varies enormously, from just one or two (for example in some beetles) to over 100 (for example in bush-crickets).

Insects can possess eyes of two types: simple eyes known as ocelli, and compound eyes. The compound eyes are the structures usually referred to as the eyes of an insect, as prominent paired structures on the top or side of the head. Each compound eye is composed of a number of separate units known as ommatidia, each of which has its own lens at the surface of the eye. An insect’s eye may be composed of just a few ommatidia, or up to tens of thousands (for example in dragonflies), and generally speaking, the more ommatidia there are, the better the insect’s vision will be. The ommatidia are visible on close examination (including in good magnified photographs of insects) as facets on the surface of the eye. Each ommatidium transmits the signal for an image to the brain of the insect, which is then turned into a composite picture. Obviously, more ommatidia allow greater detail to be resolved, and it also follows that insects are particularly good at detecting movement as an object moves from one ommatidium to another.

Antennae of a male Convolvulus Hawk-moth.

Virtually all adult insects have compound eyes, with very few exceptions (for example some scale insects), but no larvae do. Simple eyes, or ocelli, are present in larvae and in scale insects, but they are also present in insects that have compound eyes, though they are often very inconspicuous. They have no focusing mechanism and can detect no detail, but they are sensitive to light levels. Their function in adult insects is uncertain, and may be more or less obsolete, though they are more prominent in some groups such as the Hymenoptera. They are usually placed somewhere near the top of the head.

Insect mouthparts could be the subject of a whole book in themselves. They are highly variable in structure and function, and different components of the mouthparts may be greatly modified and enlarged or reduced according to requirements. Insects like cockroaches or some beetles demonstrate the basic mouthpart pattern, from which other types have evolved.

There is no internal jaw in insects, as there is in mammals, so any preparation of food for ingestion is performed by the external mouthparts. The mouthparts are located at the front of the head, and made up essentially of four parts. The top is formed by the hardened upper lip (labrum), which is actually part of the head capsule. Below this lie the paired mandibles, or upper jaws, which are heavily sclerotized and provided with powerful muscles; it is the mandibles that equate most closely with the idea of ‘jaws’ and they may be toothed, pointed and very strong. The mandibles move in towards each other from the sides, and are concerned with crushing and cutting the food.

The ferocious biting mouthparts of the Green Tiger Beetle.

Various insect mouthparts

Below the mandibles lie the paired maxillae, or lower jaws, to which are attached some segmented antenna-like appendages known as the maxillary palps. The maxillae themselves help to hold the food, while the palps have a sensory function, detecting taste and acceptability as food. The lowest part of the mouthparts is made up by the labium, or lower lip, which may carry appendages known as the labial palps.

Mouthparts such as those described above are of the biting type. However, some insects have sucking and piercing mouthparts, or various modifications and combinations of these. For example, butterflies and moths have long probosces, curled when not in use, which are used to suck either nectar from flowers or nutrient-rich liquids from other sources. Bugs, and some ‘biting’ flies like mosquitoes, have piercing mouthparts for taking in sap or blood, both of which flow out under pressure, obviating the need for a sucking capability. Many flies, such as hoverflies, have ‘suction pads’, with which they soak up liquids.

THE THORAX

Behind the head lies the thorax, usually clearly separated by a ‘neck’ or joint. The thorax is made up of three underlying segments, named (starting from the head end) as the prothorax, the mesothorax and the metathorax. Each of these segments carries a pair of legs, if all three pairs are present. Wings are borne on the mesothorax and metathorax. The shape and size of the segments varies considerably, depending to some extent on the tasks they perform. For example in flies, whose hindwings are reduced virtually to nothing, the metathorax is very small. The shape and markings of the various segments may be useful for identification.

The hardened plates, or sclerites, of the thorax all have names, though only two need concern us here. Grasshoppers (p.66) have a strongly developed pronotum, which forms a sort of shield over the thorax, extending down the sides and back to the abdomen. In the groundhoppers (p.74), the pronotum is extended back over the abdomen, or beyond it in some species and its shape is a useful aid to identification. In some bugs, especially shieldbugs (p.87), a protective plate over the mesothorax, known as the scutellum, is enlarged, forming the triangular patch between the wings, and even extending to the tip of the abdomen in some species, such as the European Tortoise Bug, Eurygaster maura.

THE ABDOMEN

The abdomen forms the remainder of the insect body. It is basically made up of II segments, but segment II is usually small or absent, and segment one is often much reduced. In many species, the other segments are clearly visible as divisions, though in some groups they are combined to form fewer divisions. Segments 8 and 9 usually bear the genitalia, which may be either inconspicuous or readily visible. In some groups, such as bush-crickets or ichneumons, there is an ovipositor, shaped according to the egglaying requirements.

The abdomen may also possess projections from the terminal segment known as cerci. These may take the form of slender ‘tails’, as in mayflies (see p.42) or stoneflies (see p.64), or be short and more robust, as in grasshoppers, or be modified into ‘pincers’, as in some dragonflies and damselflies, or earwigs (see p.84). Their function seems to be mainly sensory, rather like antennae. In a few groups, the dorsal sclerite is projected backwards as an additional tail between the two cerci; this is especially noticeable in the three-tailed bristletails (see p.38) and some mayflies (see p.42).

THE LEGS

The legs of insects are borne on the thorax, in three pairs. This is one of the distinguishing characteristics of insects as a group. As with other insect parts, there is great variation in leg structure, and some or all of the legs may be absent in some species or groups (though there are never more than three pairs). The simple basic structure, as seen in fast-running species, such as cockroaches or ground beetles, is as shown in the diagram. Some typical variations include the enlargement of the hind femora in grasshoppers and bush-crickets, for jumping; or the enlarged and powerful front legs of the Mole Cricket, used for digging. Grasshoppers use their legs as part of the system for producing their characteristic calls.

THE WINGS

More important, from several points of view, are the wings. The possession of wings is one of the features that distinguishes insects from other invertebrates, and one of the key factors in their success (see p. 4). They are also a useful aid to identification, especially in groups like the Lepidoptera, where the pattern and colour of the wings is all-important.

Structurally, wings are not limbs, but outgrowths of the thorax. Most insects, such as Lepidoptera, Hymenoptera, and Hemiptera habitually have two pairs of wings, though they are not necessarily both conspicuous nor equal in size and shape. The true flies (Diptera) have only one pair of wings, on the mesothorax; the hindwings have been reduced to two projections known as the halteres, which aid in balance. Groups such as the fleas are known to have had a winged stage at some point in their evolution, but gradually lost them as they became irrelevant to their life-style. The small group of Apterygota (wingless insects, see pp.38–41), by contrast, are believed never to have been winged.

A Cardinal Beetle about to take off, giving a clear view of the difference between its forewings and hindwings.

Insect wings can vary considerably in texture and appearance. The commonest type, found in the Hymenoptera, flies, dragonflies and other groups, is the membranous wing – the typical ‘bee’s-wing’ – transparent or translucent and very delicate. Membranous wings may be completely transparent, or they may have varying degrees of suffusion with deeper colours, such as in the Banded Agrion damselfly. In some groups, such as the scorpion flies (see p. 116), the pattern of darker marking is very useful in identification.

Membranous wings have patterns of veins within them, which may vary from highly complex to quite simple. The probable evolution of these vein patterns has been worked out in great detail, and every vein and cross-vein has been given a name or symbol. For identifying some groups and species, this pattern is crucial, but the technique is too detailed for this type of book. The only veins or cells (the blocks enclosed by veins) mentioned in the text as being useful for identification are the costa, which is effectively the leading edge of the wing, and the pterostigma (see p.46) which can be a useful aid to dragonfly and damselfly identification.

Some groups of insects have rather different wings. For example, butterflies and moths have wings that are wholly covered with scales. The colour and pattern produced by these scales is critical for their identification.

Swallowtail butterfly wing detail.

A few groups of insects, notably dragonflies and damselflies, move their wings independently during flight. This is viewed as a primitive characteristic, and more advanced insects have developed various ways of linking the wings together. This does not, however, mean that dragonflies are poor fliers; they are among the most agile of insects. The degree and the method of coupling varies in other insects, from simple overlap to complex lines of hooks like an early version of velcro.

Insects fold their wings in various different ways when at rest. This can be a useful feature in deciding which group an insect belongs to. They may be held roof-wise over the abdomen, as in the Alder Fly and some caddis-flies, or parallel and adjacent to the abdomen, as in stoneflies and most damselflies. Butterflies, mayflies and a few others hold them vertically erect, while dragonflies and some hoverflies hold them out at right angles to the abdomen (though hoverflies hold them at varying angles, or fold them away along the abdomen, depending on conditions). Some insects like the skipper butterflies or the emerald damselflies (Lestes spp.) have an in-between system, which does not fall into any of the above categories.

Insect wings at rest

How an Insect Works

This is not the place for a detailed exposition of insect biology, but a brief summary of the internal workings of an insect may be useful.

Although the body structure of insects is very different from the more familiar structure and physiology of mammals, there are many points of similarity, or features that can be readily compared, even if they have developed in a different way.

THE DIGESTIVE SYSTEM

The digestive system of insects is broadly similar to our own, with a tube that runs from the mouth to the anus. Generally, this is much less convoluted than those of mammals, but it is separated into parts which break up the food, store it, digest it, and then excrete it, in a relatively simple way.

BLOOD AND BREATHING

Insects have a blood system which accounts for much of the weight of the insect body, though it operates in a rather different way from ours. There is a heart and main artery, but most of the organs are bathed in blood which circulates freely in the body cavity, aided by additional pumps in the extremities in larger or more active insects. Its function is rather like the oil in a car engine, providing a continuous lubricating, cleaning and antiseptic system for the internal organs. It plays virtually no part in the distribution of oxygen, in contrast to ours, and is not laden with haemoglobin. (The red blood that may appear from a squashed mosquito is most likely to be your own!)

Breathing (respiration) is accomplished by a system of tubes called tracheae, which are open to the air and penetrate all parts of the body. The openings are known as spiracles, and the tubes are essentially ingrowths of the body wall, being lined with chitin. Air moves into the body by diffusion, which is adequate for small insects, and for larger ones when at rest. Larger insects require additional air-sacs, rather like lungs, which are alternately filled with air and emptied, and this pumping can be readily seen on some insects at rest as a rhythmic ‘breathing’. Even aquatic insects breathe air, and most carry a bubble of air underwater with them, which gradually becomes exhausted. A few have a system that allows oxygen to diffuse into the bubble from the water, so that it acts like an external gill.

This method of tracheal respiration, which depends heavily on diffusion rather than an efficient carrier system, is one of the primary factors that limit the size which insects can reach. Because of the inherent inefficiencies of the system, no part of the body can be very far from the surface, or it will become starved of oxygen. Very few insects in this part of the world have bodies that are more than about 20mm across the narrowest dimension, excluding the wings.

TEMPERATURE CONTROL

Insects do not possess the precise temperature control mechanism of mammals, and are therefore very dependent on the temperature of their surroundings. Groups of insects vary considerably in their dependence on warmth, and there are species that manage to remain active at quite low temperatures, especially nocturnal insects such as some moths, and powerful insects like bumble bees and some dragonflies. By contrast butterflies and many other insects are greatly affected by warmth; some butterflies will not fly when the temperature falls below about 25°C, and for others flight becomes slow and short as the temperature falls. Very few species of butterfly fly at all if the temperature falls below 14°C.

Male Common Blue butterfly warming itself in the early morning.

Although these insects have no internal control of body temperature, they have certain behavioural mechanisms that allow them to warm themselves up, or cope with cold. In cooler, but sunny, weather, many species bask in the sun, frequently angling their wings to receive maximum radiation, by holding them at right angles to the sun’s rays. Early in the morning, after a cool night, this is particularly helpful to them in allowing them to get moving before the air temperature has reached the critical level. For the naturalist, early on sunny mornings can be an excellent time to go out looking for insects, as they are still slow-moving, but often quite conspicuous as they spread themselves in the first rays of the sun.

FLIGHT AND BALANCE

Flight is of enormous importance to most insects, and is undoubtedly one of the keys to their success and wide distribution. Most insects fly by means of muscles in the thorax wall which do not directly move the wings but deform the thorax wall, causing the wings to rise and fall accordingly. In particularly strong fliers like dragonflies, the flight muscles make up about a quarter of the body weight, though in other species it is usually 10–20 per cent. The speed of the wing beat varies widely according to the weight of the insect and its manner of flight.

Insects maintain balance by constantly altering the angle of their wings as necessary. In the true flies, many of which have a poorly balanced shape, the halteres (see p.204) are highly developed balancing organs, well supplied with sensory facilities. Other insects receive the information to maintain their balance by a combination of keeping the highest light intensity on their back, general visual pattern, and information from tactile hairs on the head. Many insects are extremely agile fliers, and clearly the system is very effective.

Male Southern Hawker dragonfly in flight.

The Life History of Insects

Because of the limitations of their skeletal structure, insects are unable simply to grow steadily from being small to being fully grown, as we do. To achieve this growth, they have evolved a number of techniques, and as a result they tend to have rather complex life-cycles, with a number of different stages.

Most insects have an essentially annual life-cycle. Although some adults hibernate, they will not normally live long into their second season, and their total life-span is usually well under a year, as an adult. Almost all insects begin life as an egg (a few insects, particularly aphids, can give birth to live young, but this is a special adaptation, and is the exception rather than the rule). Eggs come in many forms and sizes, but essentially they represent a way of protecting the tiny embryo against a wide range of unfavourable conditions (such as winter cold, or drought), and they therefore have tough, resistant outer coatings. Many insect eggs have beautiful shapes and textures when seen under magnification.

Orange-tip butterfly egg.

Common Shieldbug eggs.

Large White butterfly eggs.

METAMORPHOSIS

The young insect eats or splits its way out of the egg-case and emerges into the harsh outside world. In virtually all cases, except for a few primitive or specialized insects, the young bear little resemblance to the adults of the same species, and will undergo profound changes known collectively as metamorphosis. From this point onwards, the development of insects falls into two groups, each with a distinctive pattern of development from young stages to adult. One group, considered to be the more primitive, begins to develop wings on the outside of the body from a very early stage. Each time the insect moults (see below), it gets larger, and the wings develop slightly more. In these stages, the young resemble the adults in many respects (becoming increasingly like them after successive moults), and usually live in the same places and eat similar food, albeit smaller, softer or easier to catch. Insects in these stages are known as nymphs, and the insects that undergo this type of life-cycle are called exopterygote insects (exo – outside, and pterygote – winged). The whole process is known as incomplete metamorphosis, because there is not a total change as seen in the following group; the adults differ from the young in size, and in the possession of fully formed wings and sexual organs.

Common Shield Bug eggs and mass of newly hatched nymphs.

Green Shield Bug nymph. The yellowish triangular undeveloped wings are clearly visible.

A group of immature Meadow Grasshoppers, an exopterygote insect.

Exopterygote life cycle

Endopterygote life cycle

Some aquatic insects, such as dragonflies, give the impression of a greater change because the young stages are aquatic while the adult is a totally different winged terrestrial insect. Certainly the transformation is rather more dramatic here than in, for example, the last two development stages (instars) of a grasshopper, but it is still the same process. Strictly speaking, therefore, dragonflies in their young stages should be known as nymphs, though they are often referred to as larvae.

Insects in the other group are known as endopterygote insects, and they undergo complete metamorphosis, as exemplified by butterflies. In these insects, the young are totally unlike the adults. These larvae will almost certainly have quite different feeding habits from the adults, and may occupy a wholly different ecological niche. Many longhorn beetle larvae, for example, live in rotting wood, whilst the adults are pollenfeeders, visiting roses and other flowers. The larva grows steadily, constantly shedding its skin, often changing gradually in colour, but retaining the same general form. When the larva has reached its full size, it ceases feeding and turns into a pupa (sometimes known as a chrysalis). This is essentially a protected resting phase, during which the insect undergoes a total internal re-organization in readiness for appearing as an adult. It is protected against desiccation, and to some extent against predation or parasitism, by a tough outer skin, and possibly by good camouflage. Many species pupate in the soil. The pupal stage may also serve as a means of surviving the winter, though different groups have different strategies for this.

The larva (caterpillar) of the Orange-tip butterfly, an endopterygote insect.

Pupa of an Orange-tip, just before emerging.

The adult Orange-tip butterfly.

Typical exopterygote insects with nymphal stages include bugs, grasshoppers and crickets, cockroaches, and dragonflies and damselflies. Typical endopterygote insects, with larval and pupal stages, include butterflies and moths, flies, Hymenoptera and beetles.

MOULTING

All insects from both groups, as well as the primitive wingless insects, have to undergo a process of moulting. Their chitinous external skeleton cannot simply grow with them, so insects grow a new covering under the old one, which is then split by virtue of the insect enlarging itself with the aid of air or water; the insect remains enlarged until the new skin hardens in the air, and then contracts again, leaving itself some new space for growth.

Each phase between moults is known as an instar, and the process of moulting is known as ecdysis. Some species moult numerous times, while most moult somewhere between 5 and 10 times. In species that moult only a few times, the individual phases are usually recognizable to an expert, and can be called ‘third instar’, ‘final instar’ (before adulthood), and so on.

Speckled Bush Cricket just emerged from a cast skin.

ADULTHOOD

When the fully winged adult emerges from the pupa or last nymphal instar, it usually needs to ‘pump up’ its crumpled wings to their full size and allow them to dry. Different groups do this in different ways and at different speeds, according to their particular life-style, though for almost all of them it is a vulnerable phase, and they need to do it as inconspicuously as possible. Species that emerge more or less synchronously from a limited breeding area, such as dragonflies, are especially vulnerable: once a predator such as a blackbird discovers what is happening, it can kill and eat a considerable number of insects in quite a short time. Studies have shown that this is easily the most dangerous stage of the life-cycle for many insects. Dragonflies tend to emerge at night for protection, to be ready to fly at daybreak, but they can only do this if the nights and early mornings are warm.

Four-spotted Chaser dragonfly emerging as an adult from its nymphal skin.

The adult winged insect is known as the imago. It is generally true to say that only adult insects can fly, though the mayflies (see p.42) are a curious exception. In this group, the final-instar aquatic nymph crawls out of the water on to some vegetation, and a winged insect emerges. This sub-imago can fly almost immediately (though rather weakly), and it soon reaches a sheltered place where it settles. This phase is dull in colour, owing to a downy covering, and it is known to fishermen as the ‘dun’. After settling, the mayfly moults again to produce a shinier, more strongly coloured fully mature adult (known as the ‘spinner’). Apart from this exception, and some of the wingless insects, adults do not moult or grow any more once they have become adult.

TIMING

The life-cycles of insects all follow one or other of these patterns, though they may differ in the way in which they do it. For example, some insects simply produce one generation of adults each year, and there is an exact annual cycle, varying only slightly according to weather conditions. Other species may have larvae that live, steadily growing, for many years, such as many wood-feeding beetles, or some of the larger dragonflies, though the length of time as a larva is influenced by temperature and food quality or availability. Many species may produce several generations in a year; for some, this is simply a matter of producing as many generations as possible whilst conditions are good. For others, it is a more tightly regulated system, depending on the availability of a particular food-plant, for instance. The Holly Blue butterfly has two regular generations per year; the first-generation adults emerge from the pupa in spring, and eggs are laid on the flowers of Holly. These develop, and finally emerge later in the summer as second-generation adults, which lay their eggs in the young flowers of Ivy. In many butterflies, the two generations can be rather different in colouring, and in one species, the European Map, the two generations are so different that they were originally thought to be different species.

Different insects also differ in the way in which they overwinter. Most insects overwinter as eggs or pupae, but others overwinter as larvae or nymphs, whilst quite a few hibernate in the adult phase. It depends partly on habitat, but in other cases there is no obvious reason why one or other strategy should have been adopted.

The Ecology of Insects

In recent decades, especially the last two, there has been a great upsurge in the study of insect ecology. For one thing, many of the basic anatomical and taxonomic features have by now been well worked out; secondly, study of the economic role of insects has continued apace using more sophisticated techniques; and thirdly, the great decline in some of our more conspicuous and familiar insects (and, no doubt, in many less conspicuous ones, too) has led to a need to find out what is going on in insect populations, and why it is happening.

These latter conservation-orientated studies have tended, not surprisingly, to concentrate on obvious species such as butterflies, though our knowledge of other groups has increased considerably, too. Despite their uneven nature, such studies have shown that many of these better-known insects have remarkably complex life-cycles and requirements, and many old and rather simplistic ideas about insect ecology have had to be discarded. Some generalizations about other groups of insects and their ecology can be made from specific studies, too.

EXACTING REQUIREMENTS

The Large Blue butterfly is a well-known, but nevertheless excellent, example of just how surprising an insect’s requirements and life-cycle can be. Large Blues occur in rough, grassy, flowery places, and it was well known that the larvae fed on wild thyme. Many such places exist (though they are declining), yet the butterfly was declining to the point of extinction in many areas, and more information was clearly needed. Subsequent studies revealed a remarkable story: during the first three instars, the larvae feed on wild thyme, like any ordinary caterpillar (apart from the fact that they have well-developed cannibalistic tendencies, and will quickly finish off any smaller larvae of their own species!). When they moult to the fourth instar, however, their behaviour changes; they cease feeding on the thyme, and fall to the ground, where they wait patiently until discovered by ants. This does not usually take long, as ants are abundant in such habitats, though normally only one or two species of ant are involved successfully.

The ants carry the larva into their nest, helped by the larva which makes itself as easy to carry as possible. The larva produces a sugary ‘nectar’ which the ants consume. Once in the nest, the butterfly larva begins to eat the ant larvae, whilst continuing to secrete ‘nectar’ and ant-friendly pheromones. It spends the rest of its larval life eating the ant brood, unmolested, then finally pupates in the ant’s nest in spring; the adult emerges from the pupa in the nest, and crawls to the surface, untouched by the ants. Because of its carnivorous habits, any one ant nest can only support one or two larvae, and the nests of some ant species are quite unsuitable. This means that there has to’ be a substantial population of ants just to support a small population of the butterfly, so it is hardly surprising that the species is rare and declining. Many other species of blue butterfly have similar relationships with ants, to an equivalent or lesser degree.

A wholly different example is provided by the Stag Beetle, or some of the longhorn beetles (see p.290). In these species, the eggs are laid on old wood, and the larva feeds on the decaying wood. The larger species may spend up to five years in the wood (wood is not very nutritious!), which means that successful larval sites are quite uncommon; the wood has to be decayed and soft enough to be edible (living trees are not suitable), yet not so decayed that it falls apart during the period, leaving the larvae exposed to predators. Man often plays a part here, too, by removing much dead and decaying timber. The most suitable larval sites tend to be old, but stable, boles of large trees, and most of these beetles, when they emerge as adults, also need flowers, where they collect pollen or nectar. Sites that meet both these habitat requirements are increasingly uncommon, as are the beetles that they support.

A male Stag Beetle. Its membranous hindwings are just being unfurled ready for flight.

INSECT NUMBERS

Insects may be small, but this is more than made up for by their enormous variety and huge numbers. It is often said that plants are the basis of any ecological system, as indeed they are, but insects run a close second in many ways, forming vital links in all parts of the chain. The numbers involved where insects are concerned are so large as to be almost inconceivable. In Europe alone, there are almost 100,000 different named species of insect, with the expectation that there are many more still to be described. In the world as a whole, about a million insects have been described so far, and it is certain that there are as many again waiting to be described. In France alone, there are more species of fly than there are species of mammal in the world. Generally speaking, the differences between species of insect are just as great, scale for scale, as between species of mammals and birds, and in some cases much greater.

Obviously the number of species of insect is only one aspect of their total numbers, but it gives an idea of the range of ecological niches and climatic zones that insects can occupy, as every species requires at least slightly different conditions, if only on the smallest of scales. There is also the question of total numbers. Insects vary in this respect in the same way that mammals and birds do, in that – as a general rule – the larger they are, the fewer there are of them, and the higher up the food chain they are, the lower their numbers. In addition, there are obviously insects that are ‘doing well’ in our present-day man-influenced environment, such as the House-fly or the Large White butterfly, and others that are declining, such as the Large Blue butterfly, or the Goat Moth with their demanding requirements for a declining combination of habitats.

Even the bigger insects are sometimes found together in large numbers. These are roosting 4-spotted Chaser dragonflies in an exceptionally good year for them.

If you pick up a current edition of a detailed book on birds, you will find statements to the effect that there are ‘420 pairs of Golden Eagle in Britain’, or ‘441 pairs of Dart-ford Warbler in Hampshire’. Even allowing for the fact that such estimates may be over-precise, there is simply no possibility of doing the same for insects, and in all but the case of extreme and attractive rarities, total insect populations are unknown. However, there are estimates for individual species or groups expressed in terms of the numbers per square metre or, for large species, in terms of the numbers on particular sites. For example, it is estimated that there are 2–3 million ants in a large colony, including up to 5,000 sexually active queens. Springtails may reach huge numbers, with estimates per square metre (to a depth of 30cm) varying up to 400,000 individuals. By contrast, a population of an uncommon butterfly, even in a good site, may number only 50–100 individuals, varying from year to year.

Looking at the insect world as a whole, it is clear that insect numbers are enormous and that insects pervade every aspect of ecology. Their feeding habits, and their role as prey for other animals, should be viewed in this light.

FEEDING HABITS

The range of feeding habits amongst insects is almost as diverse as the range of insects themselves, especially if you bear in mind that larvae and young nymphs may feed quite differently from adults.

The great majority of insects are plant-feeders at some stage in their lives, though this simple statement masks a great variety of feeding strategies. For example, some insects (such as aphids and many other bugs) suck the sap from the leaves or stems of plants. Some insects, such as sawfly larvae, eat virtually all of the leaf material directly, perhaps just leaving the harder veins. Other insects, such as many beetles and some hoverflies, eat rotten wood in their larval stages. Many adult insects (such as butterflies, hoverflies and longhorn beetles) visit flowers to consume either nectar or pollen as their main food source; other insects, whether larval or adult, eat mainly developing seeds and fruits. Some adult insects, such as mayflies, do not feed at all. Their feeding is all done by the nymphs or larvae.

A female Snipe Fly eats her prey while mating. The male has orchid pollinia stuck to its face.

The Currant Gall, caused by a small wasp, is one of several galls found on Oak trees.

One interesting and unusual group of plant-feeders is the gall-formers (see pp. 302–11