Key to the Identification of Insects to Order

A Few Words About KEYS

Adapted from Harold Oldroyd 1958.

Proceed directly to the Key

Keys work by a process of elimination, gradually narrowing down the number of possibilities. It is important to understand that a key is much more trustworthy in proving that your insect is not A, than in proving that it is B. It might, for instance, actually be C, a species not mentioned in the key. A key does not prove anything positive, it only suggests possibilities. People used to be faddy about the sort of key they liked, but these days the arrangement that has the greatest acceptability for simplicity and directness is the dichotomous key, so-called because at each step it asks you to choose between two alternatives. Thus:

    1..... Two pairs of membranous wings...........................................2
    Only one pair of membranous wings, the other pair
    being either hardened into wing-cases, or absent ..................29

    2...... Fore-wings and hind-wings alike............................................3
    Fore-wings and hind-wings different......................................I8

    3..............................................(and so on).

If the insect agrees with the first alternative of couplet 1, then you proceed to couplet 2; if the first alternative of couplet one is wrong then read the second alternative, if this is correct, then you proceed past all the intervening couplets, and read on at couplet 29 as indicated. Go on like this until you come to a final choice where you will be offered a probable identification:

i.e.

    35.... Abdomen with forceps......................... Earwigs (Dermaptera)
    Abdomen without forceps...................... Beetles (Coleoptera)

 

If you are lucky this will give a correct identification, but it may not. Using keys appears difficult at first but it becomes easier the more you practice and is quite rewarding. A good way to understand keys better is to make your own. Remember that the details mentioned in the key are not the real reasons why we believe an insect is any particular species: they are only pointers towards something we can recognise. The real identification is made by comparing the specimen with another specimen that is already named, or with a drawing, or with a good, detailed description. however many modern keys contain, or are appended by a sufficiently good description to allow for competent identification.

People commonly make one of two mistakes in using keys. The first is to be over-cautious, taking every word literally, and refusing to go on if the specimen does not agree exactly. Remember that insects, like humans, have their individual variations. Think of making a key that tried to classify the people present at one moment at Waterloo Station, and group them into rigid categories, without any margin of error. If you come to a point in a key at which you cannot decide which of the two alternatives fits the specimen, do not give up. Consider first if it definitely fits neither: for example, if both alternatives talk about wing-structure, and your specimen has no wings, then the likelihood is that you have reached a wrong part of the key; or else your specimen is an exceptional one that the key does not properly provide for. If, on the other hand, either alternative could be considered to fit, try each one in turn and see how you get on. If the key is a good one you will usually find that on one of the two paths you quickly gain confidence again. Here again, experience of the group will help you to know which characters mentioned are likely to be clear-cut, and which you would expect to find very variable.

Making Your Own Key

Generally speaking, the most diffficult part of a key to construct is the beginning, separating the bigger groups. Obviously, the bigger the group, the greater the variation that is likely to be found in it, and the more exceptions there will be to any definite statement that one may make about the group. Consequently, when an insect is being identified by means of a key, the first two or three steps are likely to be the most uncertain. Fortunately, experience redresses the balance, because after a little practice you will know at sight more or less where the specimen belongs, and will need to use only the last few couplets, where the differences are more definite. The second fault in using keys is to trust them too implicitly, to run down the specimen rather casually, and then to label it and put it away in a collection without checking it further. Many errors have arisen through this practice, especially when the misidentification has been published. Do remember that the key does not identify the specimen: it only gives a hint of what it might be. The correct attitude to keys--at least to keys made by other people--is to be skeptical without being defeatist. The best keys are often our own because when we read a couplet in a key we ourselves wrote, we have the advantage of a great many mental impressions that are not mentioned in the couplet, and which are not available, of course, to anyone else. That is why so often the only person who can use a particular key successfully is the author.

The arrangement of couplets illustrated above is the most common, and to my mind incomparably the best one. The advantage of having the two alternatives side by side, so that you can weigh up your specimen against each of them, far outweighs the benefits claimed for other arrangements. Many entomologists, particularly those of an older generation, prefer the following style:

01. (I6) Two pairs of membranous wings ...
02. ( 9) Fore- and hind-wings alike ... ...
03. ( 7) . . .
09. ( 2) Fore-wings different from hind-wings
I6. ( I) Only one pair of membranous wings...
           . . . (and so on)

If 1. is correct you proced to 2. but if it is incorrect you procede to16 to read what would have been the second alternative in a dichotomous key.

This kind of key is often significantly called a 'Table', and is said to show the grouping of the insects better, while the numbers in brackets, referring to the other half of each couplet, make it easier to trace a way through the key, both forwards and backwards. Bracketed numbers can, of course, be used in the other type of key to help in retracing one's way. The need to turn over, sometimes, many pages to find the alternative character in a 'Table' of this kind, makes the operation into too much of a hurdle race.

Beware of keys in which more than two alternatives are presented at one time, especially if the author does not bother to indicate this very clearly. This is bad practice, because it defeats the first object of the dichotomous key, which is to proceed by a series of clear choices between simple and mutually exclusive alternatives. Occasionally, it is true, the insects fall naturally into three or more co-equal groups: e.g. if the legs can be red, or black, or yellow. But such a choice can nearly always be set out in couplets with a little more trouble, and the slightly greater printing space is more than compensated for by the assistance it gives the reader.

Another thing to look out for in keys--and to avoid in your own--is 'leapfrogging', like this:

    40. Femora greatly swollen......................................41
    Femora slender..............................................72
    4I. Femora with spines beneath...............................71
    No ventral spines on femora...........................85
    42. Eyes hairy ........................................................48
    Eyes bare.......................................................43

Of course, if you read this carefully, you will have no trouble, but keys are meant to give help in quick identification, and you should always try to keep to the logical order. Remember keys should be designed to be used by someone with no previous experience of the species or groups involved. The construction of keys is a very searching test, not only of how much you really know about a group of insects, but also of whether your knowledge has been assimilated by a tidy and logical mind. When an author publishes a complicated and disorderly key, you will find that the rest of his work shows evidence of a chaotic and turbulent state of mind.

 

 

Constructing and Adapting Keys

For all that we have just said, do not think that making keys is only for the professional entomologist. If you have studied a group of insects, it is well worth while to make a key to them, for your own use, even if you have no idea of publishing it. It is even more useful to be able to modify an existing key and adapt it to your needs: for example, to take a key to the European genera and species of a family and to abstract from it those which occur in Britain; or to combine two or more short keys into one that is more comprehensive. You may need to do this if you do not own the books concerned, but have to borrow them from a library, and return them after a limited time. Then you can pick out just the parts you need, without having to copy out a great deal that is of no value to you. For all key-making, I suggest you use a system based upon a card-index, using standard cards 5 X 3 in. Suppose we begin by copying a short key from a volume borrowed from a library. Write each couplet on a separate card, and stack the cards in a filing cabinet, or in one of the small portable index-boxes (called, for some mysterious reason, a 'trial outfit'); or at worst, hold the cards together with a rubber band. Then if you want to add more names to the key, run each one down as far as it will go, and add another card at that point, using a decimal system of numbering, thus:

The use of cards avoids all the crossing out, cutting-up and pasting together that is needed if you copy the original key on to sheets of paper. Such a card-key can be expanded indefinitely, each time just for the labor of adding a new card, an making a small alteration to the previous one (for this reason it is often better to write in pencil rather than in pen). To put a card between 21.5 and 22 you may number it 21.6; if you need one between 21.5 and 21.6 then it becomes 21.55, and so on like the Dewey Decimal System used in libraries. So long as the key is only for private use, and is liable to be altered, the decimal numbers can remain. When you want to publish it, you go through from the beginning, numbering the cards in order in red ink and, of course, not forgetting to make the corresponding alterations to the right-hand numbers.

HOW TO DISTINGUISH THE ORDERS OF INSECTS

At this point, it may be useful to present a key to the Orders of Insects, which will serve both as an example of a key in use, and as a first step in identifying a specimen. The key is in two parts, one for insects with wings, and one for those without. Do not take these keys too literally. As we have said already, the big groups are the most difficult to define in a key, because they include such a range of variation, but they are also the groups that one can most easily learn to know by sight. The present keys are introductory, not exhaustive, and will not track down obscure and difficult species; for these a text-book of entomology will be needed.

Key to the Orders of Insects

Notes:-
Tarsi (singular tarsus) are the last major unit of an insects leg see Fig 5.
Cerci. (singular: cercus) are the paired appendages, often very long, which project from the tip of the abdomen in many insects.
Cornicles are the pair of small tubular outgrowths which occur on the hind end of the abdomen of an aphid they, tend to point upwards and backwards from a little way forwards or the tip of the abdomen.

    1a. Insect with wings...............................................................................................2
    1b. Insect without wings........................................................................................27

    2a. Insects with four wings (two pairs).....................................................................3
    2b. Insects with only two wings (one pair)..............................................................25

    3a. Wings covered with scales............................Butterflies and Moths Lepidoptera
    3b. Wings not covered with scales, though they may be hairy..................................4

    4a. Fore-wings partly or entirely horny or leathery and used as covers for
    hind-wings often much narrower than hindwings............................................5
    4b. Both pairs of wings entirley membranous (flexible) and used for flying...............12

    5a. Mouth-parts tube-like, adapted for piercing and sucking.. True Bugs Hemiptera
    5b. Mouth-parts adapted for biting and chewing.....................................................6

    6a. Fore-wings and hind-wings with veins, hind-wings stiffer and harder than
    and serving as covers for hind-wings..............................................................7
    6b. Fore-wings without veins, and modified into hard, horny cases for hind-wings...10

    7a. Body dorsoventrally flattened....................Cockroaches Dictyoptera; Blattodea
    7b. Body rounded or quadrate in section..................................................................8

    8a. Forelegs raptorial, adapted for grasping and holding..................Preying Mantids
    ...............................................................................Dictyoptera; Mantodea
    8b. Forelegs not raptorial.........................................................................................9

    9a. Prothorax as large as or larger than meso and meta thorax, hind legs generally
    enlarged and adapted for jumping........Grasshoppers and Crickets Orthoptera
    9b.Prothorax smaller than meso and meta thorax, legs normally similar in thickness,
    if hind legs enlarged then not used for jumping.............Stick-Insects Phasmida

    10a. Fore-wings short.............................................................................................11
    10b.Fore-wings as long as, or nearly as long as abdomen the 2 wings may be joined
    where they meet along the animals back and hence never used for flying..Beetles
    .....................................................................................................Coleoptera

    11a. End of abdomen with characteristic pair of forceps like cerci...................Earwigs
    ...................................................................................................Dermaptera
    11b. End of abdomen with out characteristic forceps like cerci........................Beetles
    .............................................................................Coleoptera; Staphylinidae

    12a. Wings narrow and without veins, but fringed with long hairs. Very small insects,
    about 5 mm in length ........................................................Thrips Thysanoptera
    12b. Wings more fully developed, and with veins present........................................13

    13a. Hind-wings noticeably smaller than fore-wings.................................................14
    13b. Hind-wings similar in size to or larger than fore-wings.......................................19

    14a. Abdomen with two or three long 'tails'. Fore-wings with a large number of
    cross-veins, making a net-like pattern......................... Mayflies Ephemeroptera
    14b. Fore-wings with fewer veins, not forming a net-like pattern,
    usually without 'tails' .......................................................................................15

    15a. Wings obviously hairy. Mouth-parts very small, except forpalpi
    .....................................................................................Caddisflies Trichoptera
    15b. Wings not obviously hairy, though tiny hairs can be seen under the microscope.....16

    16a. Mouth-parts well developed and adapted for biting and chewing........................17
    16b. Mouth-parts tube-like, adapted for piercing and sucking.........Aphids; Cicadas etc
    ....................................................................................Hemiptera; Homoptera

    17a. Very small insects, soft-bodied, mostly less than 6 mm. in length. tarsi with only
    two or three segments....................................................................................18
    17b. Often much bigger, wasp-like or bee-like insects; or if very small, then
    hard-bodied, with abdomen narrowed at its base into a petiole, or 'waist'. tarsi
    with four or five segments.......... Bees, Wasps, Ants and Sawflies Hymenoptera

    18a. Antennae with 9 segments only...................................................... rareZoraptera
    18b. Antennae with 12 to 50 segments..........................Bark or Book Lice Psocoptera

    19a. Tarsi with three or four segments only ...............................................................20
    19b. Tarsi with five segments....................................................................................23

    20a. Tarsi with 3 segments only; first segment of anterior (front) legs greatly swollen
    ................................................................................Webspinners Embioptera
    20b. Tarsi with 3 or 4 segments, if 3 then first segment of anterior legs not swollen....21

    21a.Wings with few cross-veins, fore-wings differently shaped to hind-wings which
    are greatly expanded posteriorly......................................Stoneflies Plecoptera
    21b.Wings with numerous cross veins, fore- and hind-wings usually very similar in
    shape,though hind-wings occasionally enlarged posteriorly..............................22

    22a. Small insects, generally much less than 1 inch. (25 mm.) in length with long
    antennae, and with wings folded flat over body..................... Termites Isoptera
    22b. Generally longer than 1 inch., with very short antennae. Wings held away
    from body when at rest.................................................... Dragonflies Odonata

    23a. Mouth-parts prolonged into a beak. ............................Scorpionflies Mecoptera
    23b. Mouth-parts short...........................................................................................24

    24a. Most of the veins in forewings divide or fork just before they reach the wing edge,
    hind-wings broader than fore-wings at least at base ...........Alderflies, Snakeflies
    ..................................................................................................Megaloptera
    24b. Few or no veins in the forewings fork immediatley before the wing edge,
    hind-wings similar to fore-wings....................................Lacewings Neuroptera

    25a. Hind-wings absent or reduced knob-like organs (called halteres)......................26
    25b. Forewings absent or reduced to knob-like organ.................Stylops Strepsiptera

    26a. Hind-wings reduced or modified to knob-like organs (called halteres) Mouth-parts
    of various forms ..... .............................................................True Flies Diptera
    (Also males of Homoptera, family Coccidae, but these are very rare)
    26b. Hind-wings entirely absent; no halteres. ..............Some Mayflies Ephemeroptera

    27a. Some segments with jointed legs, which can be used for movement..................28
    27b. No jointed legs; or if these are present and visible, then they are enclosed
    in membrane, and cannot move...................Larvae and Pupae of Endopterygota
    ...................................(You will need specialised keys to get these to order)

    28a. Animals found living as parasites on warm-blooded animals, or found closely
    associated with them i.e. in their nests or dens................................................29
    28b. Animals not found living as parasites on warm-blooded animals: either freeliving,
    or parasitic on other insects, snails etc............................................................34

    29a. Body flattened from side to side, hard and bristly, with strong legs, jumping insects,
    found on birds and mammals...............................................Fleas Siphonaptera
    29b. Insects not as above, body either rounded or flattened from above ..................30

    .

    30a. Mouth-parts adapted for biting and or chewing.................................................31
    30b. Mouth-parts adapted for piercing and or sucking..............................................32

    31a. Posterior end of the body with cerci. Found on bats and small rodents in tropical
    environments only.............................................. Parasitic earwigs Dermaptera
    31b. Posterior end of body without cerci. On birds or mammals all over the world
    ................................................................................Chewing lice Mallophaga

    32a. Flattened, rather spider-like insects, with head fitting into a notch on thorax, and
    with antennae not visible. Claws hooked.............Louseflies and Batflies Diptera
    32b. Not spider-like. Antennae clearly visible .........................................................33

    33a. Snout (proboscis) short, unjointed. Body long and narrow. Tarsi of legs with one
    large, hooked claw. Permanent parasites of birds and mammals......Sucking lice
    ........................................................................................................Anoplura
    33b. Snout (proboscis) longer, jointed. Body more oval. tarsi with two small claws, not
    hook-like. Only temporary parasites.........................Wingless bugs Hemiptera

    34a. Terrestrial: living on dry land, or on animals other than mammals and birds........35
    34b. Aquatic: mostly nymphal forms of terrestrial insects..........................................60

    35a. Mouth-parts not visible. Abdomen with appendages on some of the abdominal
    segments, or with a forked 'spring' near tip....................................................36
    35b. Mouth-parts clearly visible..............................................................................39

    36a. Abdomen with six segments or fewer, usually with a forked appendage ('spring')
    near tip. No long bristles at tip of abdomen................... Springtails Collembola
    36b. Abdomen with nine or more segments. No spring, but several segments have
    simple appendages.......................................................................................37

    37a. Cerci present, sometimes appearing as clasping forceps..................................38
    37b. No cerci................................................................................................Protura

    38a. A central 'cerciform tergum' projects between the cerci giving the appearance
    of 3 'tails' ......................................................3-Pronged Bristletails Thysanura
    38b. No central 'cerciform tergum', hence having the appearance of 2 'tails'.....
    ..........................................................................2-Pronged Bristletails Diplura

    39a. Mouthparts mostly adapted for piercing or sucking..........................................40
    39b. Mouth-parts not as above, adapted for biting and or chewing..........................44

    40a. Body covered with scales and or dense hairs........ Wingless Moths Lepidoptera
    40b. Body bare, or with few scattered hairs ...........................................................41

    41a. Almost all of thorax that is visible above is composed of the middle segment, the
    mesothorax: prothorax and metathorax both small and hidden.............................
    ...............................................................................Wingless True flies Diptera
    41b. Mesothorax and metathorax about equally developed. Prothorax also
    is usually visible from above...........................................................................42

    42a. Snout (proboscis) small, cone-shaped. Body long and narrow. Claws usually
    absent.............................................................................Thrips Thysanoptera
    42b. Snout (proboscis) longer, jointed. Body more or less oval. Claws present.........43

    43a. Proboscis arising from front part of head. Abdomen without cornicles near tip
    ...............................................................................Wingless Bugs Hemiptera
    43b. Proboscis arising from hind part of head. Abdomen often with two cornicles
    at or near its tip .............................................Aphids Hemiptera; Homoptera

    44a. Abdomen with false or pro-legs, which are fleshy, and different from the
    jointed legs of the thorax. Caterpillar-like......................................................45
    44b. Abdomen without any kind of legs, only thorax has legs...................................47

    45a. Five pairs of prolegs, or fewer, with minute hooks (crochets); none on the1st or
    2nd abdominal segments...........................................Caterpillars Lepidoptera
    45b. Six to ten pairs of prolegs, always with one pair on the 2nd abdominal segment.
    No crochets present...............................................................................................46

    46a. Head with a single ocellus (small eye) on each side.............. Larvae of Sawflies
    ................................................................................Hymenoptera; Symphyta
    46b. Head with several ocelli on each side................. Larvae of Scorpionflies
    .....................................................................................................Mecoptera

    47a. Antennae short and indistinct. Larvae..............................................................48
    47b. Antennae long and distinct. Adult insects.........................................................50

    48a. Body Caterpillar-like.......................................................................................49
    48b. Body not caterpillar-like........................... Larvae of some endopterygote insects
    ..............................................................................Neuroptera or Coleoptera

    49a. Head with six ocelli on each side of headsome ..............Caterpillars Lepidoptera
    49b. Head with more than six ocelli on each side..............Larvae of some Mecoptera

    50a. Abdomen with a pair of movable forceps like cerci at tip...................Earwigs
    .....................................................................................................Dermaptera
    50b. Abdomen without such forceps........................................................................51

    51a. Abdomen strongly constricted at base into a 'waist'. Sometimes antennae are
    bent into an elbow...............................Ants and wingless Wasps Hymenoptera
    51b. Abdomen not constricted into a waist...............................................................52

    52a. Head prolonged underneath body into a long beak, which bears mandibles at its
    tip.... ..........................................................................Scorpionflies Mecoptera
    52b. Head not prolonged into a beak........................................................................53

    53a. Tiny soft insects ................................................................................................54
    53b. Fairly small, to very big, usually hard-bodied insect............................................55

    54a. Cerci absent................................................... Booklice and Barklice Psocoptera
    54b. Cerci present.......................................................................................Zoraptera

    55a. Hind-legs enlarged for jumping..................................... Grasshoppers/Crickets
    .......................................................................................Saltatoria; Orthoptera
    55b. Hind-legs not enlarged for jumping ...................................................................56

    56a. Tarsi of legs with four segments. Pale, soft-bodied insects living in wood or soil.
    ..............................................................................................Termites Isoptera
    56b. Tarsi of legs with five segments. More highly coloured insects............................57

    57a.Body dorsoventrally flattened.....................Cockroaches Dictyoptera; Blattodae
    57b. Body not dorsoventrally flattened rounded or squarish in section........................58

    Cerci long, containing 8 segments, eyes reduced or absent ................ Grylloblattodae
    Cerci not as above, eyes well developed...................................................................59

    59a. Fore-legs modified for grasping and holding, predatory....Dictyoptera; Mantodae
    59b. Fore-legs not so modified................................................Stick Insects Phasmida

    60a. Mouth-parts adapted for piercing and sucking. ................Nymphs of Water-bugs
    .....................................................Hemiptera and larvae of some Neuroptera
    60b. Mouth-parts adapted for licking and chewing.....................................................61

    61a. Body enclosed in a case made of pebbles, sand and debris....Larvae of Caddisflies
    ......................................................................................................Trichoptera
    61b. Body not enclosed in such a case......................................................................62

    62a. Abdomen with external gills...............................................................................63
    62b. Abdomen without external gills..........................................................................64

    63a. With two or three long processes at tip of abdomen, traces of wing-cases may be
    visible in later instars............................... nymphs of Mayflies Ephemeroptera
    63b. Only one process at tip of abdomen, and no wing-cases visible.............Alderflies
    ................................................................................Megaloptera; Sialioidea

    64a. Head with a 'mask', bearing the jaws which is capable of being extended far
    forwards of the insect's body..........................Nymphs of Dragonflies Odonata
    64b. Head without such a mask..............................................................................65

    65a. With long antennae; and long filaments at tip of abdomen.....Larvae of Stoneflies
    .......................................................................................................Plecoptera
    65b. Without such filaments...........................................Larvae of Beetles Coleoptera

    Identification of the Lower Catagories of Insects

    The preceding 'Key to Orders' makes use of broad distinctions, such as 'mouth-parts adapted for chewing', and 'without any jointed legs'. Ordinary language will express most of these, without the need for a special vocabulary. When we come to try to distinguish between families, genera and species, we have to go into greater detail, and so have not only to have more knowledge of the structure of the insects, but also to have to have special names for many details for which there are no names in common language. There is such a great range of structure through the Class Insecta that it is impossible to give a simple account of all the structures that may be used in keys.

     

     

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