This is the second diary on the diversity of arthropods. The goal of these diaries is to provide a resource that can be used in conjunction with the bugguide website to identify terrestrial arthropods, particularly insects. In order to document biodiversity and to correlate changes in biodiversity with environmental change we need to know what we're looking at.
The first diary was a survey of the major groups with arthropods with instructions on how to identify terrestrial arthropods to subphylum. The diversity within two of the subphyla, Chelicerata and Myriapoda was partially explored to level of orders. This diary begins the examination of the diversity of the Hexapoda, the largest of the Arthropod subphyla, and the one that dominates terrestrial arthropod diversity. It is the subphylum that contains the insects and the one that will be the primary focus of this series. This initial diary will look at the classes of the Hexapoda and the Insect Orders.
Hexapoda Body Form
The term hexapoda means six legs and this is one of several characteristics of this subphylum. The Hexapods have a body divided into three main regions: a head with a single pair of antennae, a thorax with three pairs of walking legs, and an abdomen with no appendages other than genitalia and possibly cerci (sensory structures) at the back end.
The vast majority of the Hexapoda are Insects (Class Insecta). There are three groups (usually considered orders) of Hexapods that fall outside of the Insects. All three are unlikely to be observed by the average person. These groups are considered Insect Orders in older books but now would generally each be considered its own Class. All of them are small to very small animals that lack wings and are usually found in the soil, leaf litter or under objects. The Protura, which Bugguide charmingly refers to as Coneheads, are tiny (as in look at with a microscope tiny) things that lack antennae. I've never seen one except on a prepared microscope slide. The Diplura are the only one of these three groups that has individuals large enough to be examined without a microscope. They are similar to the Silverfish (see order Zygentoma, below) but only have two structures extending off the back of the abdomen. They are found in the soil or under objects and are small to medium-sized hexapods.
By far the most diverse and abundant of the three non-Insect classes of Hexapoda are the Collembola or springtails. These are very common in soil and leaf litter and are often present in huge numbers. However they are very small and often not noticed even when they are right under your nose.
A characteristic that unites these three classes is that they are endognathous, that is their mouthparts are encased in a pouch. In contrast the insects are ectognathous and their mouthparts are clearly visible.
Class Insecta
The survey of the insect orders will consist of three parts: 1) an evolutionary summary of the insects showing, in a very general way, where the orders fit within the pattern of insect evolution, 2) an informal 'key' to identify insects to order, and 3) a brief description of each order organized by likelihood of encounter.
Part One: An Evolutionary Overview
The insects can be split into a number of smaller groups based on several major evolutionary innovations (wings, ability to fold wings, larval and pupal stages). Generally each of these splits has divided the insects into two groups, one large group that is united by the innovation and a number of small groups that lack the innovation. It is important to note that the names are generally descriptive rather than definitional. Being wingless in the context of this classification does not mean simply not having wings, it means being a member of a group that has never had wings.
Apterygote Insects - These are insects that lack wings and they branched off the main insect lineage prior to the evolution of wings. There are only two orders of very similar insects: Microcoryphia (Bristletails) and Zygentoma (Silverfish). In both of these groups immature stages look just like smaller versions of the adults.
All the other insect orders are known as Pterygote Insects. As mentioned above many of them are wingless (some entire orders such as the fleas are wingless) but it is clear from other aspects of their biology that they must have had winged ancestors.
The Pterygote Insects are divided into two groups: The Paleoptera and the Neoptera.
The Paleoptera (meaning ancient wings) are insects that lack the ability to fold their wings up when at rest and have to hold them either outstretched to the sides or vertically over their backs. Again there are only two orders living: the Ephemeroptera (Mayflies) and the Odonata (Dragonflies and Damselflies). Both of these orders have aquatic immature stages called naiads. They are fairly to similar to the adults in form except for the lack of wings and the abdomen is a somewhat different shape due to the presence of gills.
The Neoptera are all the other orders and contain insects that can fold their wings over their backs. Some insects that have evolved very large wings (butterflies and some moths for example) have lost the ability to fold their wings.
It is convenient, although slightly inaccurate, to divide the Neoptera into three major groups: the Orthopteroid insects, the Hemipteroid insects, and the Endopterygota.
The 'Orthopteroid' insects is a large group of orders that at one time were considered to be closely related. Evidence has accumulated that these orders are not necessarily all closely related but have been lumped together because they all lack the characteristics of the other two groups. The classification of these orders varies considerably in different schemes. The 'Orthopteroid' orders in the bugguide classification are: Orthoptera, Phasmida, Blattodea, Mantodea, Dermaptera, Plecoptera, Zoraptera, Embiidina, and Notoptera. With the exception of the Plecoptera the juvenile stages resemble the adults but lack wings.
The Hemipteroid insects includes only three orders: the very large order Hemiptera, the Psocodea, and Thysanoptera. Most members of this group have sucking mouthparts for feeding on fluids. Juveniles resemble wingless adults for the most part.
The Endopterygota are all the insects that have larval and pupal stages. The larvae have a completely different body form than the adults and undergo a pupal stage in which the body is completely reorganized. This group includes over 80% of all insect species in the orders: Megaloptera, Rhaphidioptera, Neuroptera, Coleoptera, Mecoptera, Lepidoptera, Trichoptera, Diptera, Siphonaptera, Strepsiptera, and Hymenoptera.
Part Two: Identifying An Insect to Order
Traits to look for in identifying insects to order.
1. Wings - 0 vs 1 vs 2 pairs, membranous vs thickened vs covered in hair or scales, relative size of front and hind wings, position of wings when animal is not flying.
2. Mouth parts - beak like (sucking) vs. jaw-like (chewing) vs. spongelike (lapping) vs long and coiled
Head of a hemipteran showing sucking mouthpart underneath
3. Antennae - length and structure (this is more useful in identifying families within orders but can be diagnostic for some orders such as the Odonata).
4. Caudal appendages - structures extending off the rear of the abdomen. Number, structure, and size.
There are a couple other useful traits that can be used but they generally require the use of a microscope (or at least a hand) lens and a dead specimen although if you get really good photographs... These traits are: the number of tarsal segments (parts of the foot) and the number of antennal segments. I have not included them here although if anyone is interested I can add them in later.
Identification 'Key' for Insect Orders.
Caveats
1. A number of small orders that are unlikely to be encountered are excluded. These are listed and described at the end of the diary.
2. The key won't work for rare forms that are atypical of their order.
3. This is for adult insects or nymphs (juveniles that resemble adults( only. Larval insects will be covered later.
4. This is not a traditional dichotomous key but rather a series of lists you can use to compare to your specimen.
Answer the following question for your specimen.
Does your specimen have wings and, if so, what kind?
A. Specimen does not have wings
B. Specimen has membranous wings (translucent or transparent) or wings that are covered in colored scales or small hairs. In other words wings are not thickened or hardened.
C. Front pair of wings is thickened/hardened, appearing leathery or shell-like, covering the hind pair of wings.
Go to the appropriate box below based on your answer.
A. Wingless insect.
Is the insect an external parasite of a bird or mammal? If it is very active with long legs and is extremely flattened from side to side then it is a flea (Order Siphonaptera). If it is flattened in the other plane (i.e. top to bottom) and has short legs it is a parasitic louse (Order Psocodea, parasitic lice are the Phthiraptera).
Is the insect elongate, ground living, with three long filaments extending out its back end? If so it is a member of the Order Microcoryphia (Bristletails) if it has large compound eyes or the Order Zygentoma (Silverfish) if it has small or absent compound eyes.
Very elongate, sticklike insects with long legs and chewing mouthparts. Found on vegetation and can be very large - Order Phasmida (Walking sticks).
Insects of the following orders have juvenile stages that closely resemble the adult stage but are wingless. Many species in these orders also have wingless adults. All should be recognizable by their distinctive body forms. (see the box for option C for more details). Grasshoppers and their relatives (Orthoptera), Mantises (Mantodea), Cockroaches (Blattodea), and Earwigs (Dermaptera).
Beaklike, sucking mouthparts, usually found on vegetation or in or near freshwater. Shape and size highly variable - Order Hemiptera (bugs).
Aquatic, large head with chewing mouthparts and long legs, only emerging into the air to moult into adult. Stocky body with no tail-like projections - Order Odonata (Dragonfly juvenile). Slender body with three tail-like gills with short antennae and no gills elsewhere - Order Odonata (Damselfly juvenile). Slender body with three tail-like gills as well as gills lining the sides of abdomen and longer antennae - Order Ephemeroptera (Mayfly juvenile). Body slender to moderate with two tail-like projections - Order Plecoptera (Stonefly juvenile).
Pale, soft bodied insects found in soil or wood. - Order Blattodea (Termites)
Termites on the move - note the two soldiers on the lower right with the 'nozzle-heads'
Hard-bodied insects, sometimes covered in hair. Antlike or wasplike - Order Hymenoptera (Ants, Bees, and Wasps)
B. Membranous or other non-hardened/leathery wings
Insect has large wings that are covered in minute scales (appear as dust if handled), mouthparts are a long, coiled tube. Order Lepidoptera (Butterflies and Moths)
Insect is moth-like but has wings that are covered in hair. Antennae are at least as long as the body. Usually near water. Order Trichoptera (Caddisflies)
close up of above showing hairs (blurry)
Insect has a single pair of membranous wings (hindwings are reduced to a pair of very small, mushroom shaped structures called halteres) Mouthparts are sucking or lapping - Order Diptera (Flies)
Wings held vertically above body when at rest, hind wings noticeably smaller than front wings, two or three long filaments extending from rear of abdomen. Usually near water - Order Ephemeroptera (Mayflies).
Front and hind wings similar in size (Damselflies) or hind wing slightly shorter and broader than front wing (Dragonflies), either held vertically above body (Damselflies) or out to the sides (Dragonflies) when at rest. Antennae very short and bristle-like, eyes large, abdomen long and slender. Most often near water although can wander widely while foraging - Order Odonata (Dragonflies and Damselflies).
Hind wings shorter and broader than forewings. Wings folded flat on the back while at rest. Two cerci (sensory organs) project from back of abdomen (often hidden by wings). Antennae are threadlike but not as long as the body. Weak fliers usually found near streams - Order Plecoptera (Stoneflies)
Front and hind wings similar in size, head with a long beak-like snout (chewing mouthparts at the end of the snout) - Order Mecoptera (Scorpionflies).
Small, soft-bodied insects with two pairs of long narrow wings that are similar in size and shape. Usually seen flying in swarms. Order Blattodea (Isoptera) (Termites).
Front wings clearly larger than hind wings, wings held roof-like above the body when at rest. Sucking mouthparts, body size and shape highly variable - Order Hemiptera (Most suborder Sternorrhyncha -Aphids, whiteflies and relatives, also Cicadas)
Hard-bodied insects usually with a constriction in the body (waist) between the thorax and the abdomen (wasp-like). Body is sometimes hairy (bees). Chewing mouthparts. Order Hymenoptera (Ants, Bees and Wasps).
The last three orders in this section all have very large membranous wings that are held roof like over the body. Wings have numerous veins, particularly cross-veins. Antennae are threadlike (some have an enlarged tip). Have elongate bodies.
Medium to very large insects, wings often at least partially colored. Hindwing somewhat wider than forewing. May have very large mandibles. Usually found near water. Order Megaloptera (Dobsonflies, Alderflies, and Fishflies).
Midsize, elongate insects, with a long 'neck' (front part of thorax). All legs similar in size. Found only in the west. Order Rhaphidioptera (Snakeflies).
Small to moderately large insects. Do not have the elongate front part of the thorax (except for Mantidflies which have front legs modified for prey capture at the very front of the thorax). Some resemble damselflies but have very different antennae and fold their wings when at rest. Order Neuroptera (Lacewings, Mantisflies, Owlflies, Antlions).
Option C - Insects with hardened or leathery front wings (wings in some of these forms are often very short).
Wings frequently quite short, leaving abdomen exposed, end of abdomen has two forceps-like cerci. Elongate body, chewing mouthparts, found on the ground or on vegetation. Order Dermaptera (Earwigs).
Flattened, oval body. Front Wings have veins and overlap. Long antennae. Head concealed from above by thorax. - Order Blattodea (Cockroaches)
Elongate body with overlapping front wings with veins. Front part of thorax elongate with modified front pair of legs for grasping prey. Order Mantodea (Mantises)
Hind legs modified for jumping. Front wings with veins, held rooflike above body (grasshoppers and katydids) or flat on back (crickets). Order Orthoptera (Grasshoppers, crickets, and their relatives).
Very elongate body with short wings and long legs. Sticklike - Order Phasmida (Walkingsticks) - Most North American species are wingless.
Front wings without veins, range from leathery to hardened and shell-like. Wings meet in a straight line down the middle of the animal. Chewing mouthparts. Order Coleoptera (Beetles)
Front wings with veins, held roof-like above body, meet in straight line in mid-line. Sucking mouth parts. On vegetation. Order Hemiptera, (Suborder Auchenorrhyncha - leafhoppers, plant hoppers, etc.)
Front wings leathery at base and membranous at tips. Wings cross giving the back of the animal a characteristic X pattern. Sucking mouthparts. Usually in or near water or on vegetation. Order Hemiptera (Suborder Heteroptera - True Bugs).
Part Three: Descriptions of Individual Orders, Grouped by Diversity and Likelihood of Encounter.
This section is not evolutionary in its organization. It provides basic information on all of the insect orders, organized into groups based on how likely the average person is to encounter them. This is obviously an imprecise classification and is highly biased by my own personal experience.
Group 1 - The Megadiverse Orders. These five orders account for the great majority of all known insect species and occur pretty much world wide where ever there are insects. In most cases the conspicuous species that we notice are the 'tip of the iceberg', with huge numbers of less conspicuous species also sharing the local environment with us.
Order Coleoptera (Beetles). This is the most diverse group of insects and is notable for the diversity of ecology amongst its members. Adult beetles have their fore wings modified to hardened protective casings called elytra. Larval beetles are very diverse in form but always have a well developed head. Beetles are not as easily characterized as some other groups of insects in terms of their way of life. Adults can be recognized by the elytra meeting in a straight line down the animal's midline and by the complete absence of veins in the elytra. The insects most likely to be confused with adult beetles are some members of the Hemiptera. They can be distinguished by the type of mouthpart (beetles have chewing mouthparts).
Order Hymenoptera (Ants, Bees, Wasps). This group is characterized by two pairs of membranous wings and hard body which usually has a well defined constriction between the thorax and the abdomen. Larvae are mostly maggot-like and require either parental care or placement within a food source (parasitism or plant galls). This is an enormously important group which contains a large number of insect parasitoids and species that live in complex social groups.
Order Lepidoptera (Butterflies, Skippers, and Moths). Larvae are active herbivores (plant eaters) while adults are mostly pollinators. The mouthparts of the adults are modified to a long coiled tongue for extracting nectar from flowers. The butterflies are actually a fairly small minority of species in this order. Most species are moths, which are defined basically as any Lepidoptera that are not butterflies or skippers.
Order Diptera (Flies). Larvae are limbless and essentially headless and are usually associated with a moisture rich environment (bodies of water, decaying plant or animal matter, etc.). Adults are mostly active, agile fliers. They have lost their hind wings which are modified to form halteres, structures that function as gyroscopes to allow flies to maintain their orientation. There are two common body shapes in flies (there are certainly quite a few exceptions). One form is mosquito-like, with a slender body and longish antennae. Many of these flies are nocturnal and have larvae that live in ponds and streams. The other form has a more robust body (house-fly like) with very short, bristle like antennae and large eyes. They are usually diurnal and the larvae live in a variety of habitats.
Order Hemiptera (Bugs, hoppers, aphids, etc.). This is an order of insects in which the juvenile stages are nymphs (resemble adults) rather than larvae. They all have mouthparts that are modified to form a beak through which they suck fluid food. The majority of the species are plant feeders and consume the vascular fluids of plants. Some, particularly aquatic species, feed on other animals and some are external parasites of large animals. This order is somewhat unwieldy and is split into three groups that are fairly distinct from one another. The Heteroptera are the 'true bugs' in which the fore wings have thickened leathery bases and membranous tips. They include a large number of plant feeding species as well as many predatory species. All of the aquatic species are predators on other animals as well as some terrestrial species. The other two groups of Hemiptera, the Sternorryncha and Auchenorryncha are both exclusively terrestrial plant feeders.
Group 2 - Conspicuous but less diverse Orders These orders are commonly seen insects but have substantially fewer species than the ones listed above. In many cases they are large and conspicuous or they live in close association with humans.
Order Odonata (Dragonflies and Damselflies). This is a very familiar group of insects because most species are quite large and conspicuous. Active, diurnal insects that are most commonly found near water. Both the terrestrial adults and the aquatic juveniles are predatory. Divided into two distinctive groups the dragonflies and the damselflies.
Order Orthoptera (Grasshoppers, Crickets, Katydids, etc.). This is the largest order in this section and is really a 'mini-megadiverse' order. Again these are very well known insects. They are very common and include species found in close association with humans and important agricultural pests. The hind limbs are enlarged for jumping. The majority of species are plant feeders but many are carnivores or scavengers. Perhaps because of their jumping ability, winglessness or reduced wing size are very common in this order. This order is also known for the loud mating calls made by many species using their modified front wings.
Order Blattodea (Cockroaches and Termites). These two groups are not commonly associated with one another by non-entomologists but termites are really just highly social cockroaches! Cockroaches are flattened insects with leathery forewings that lay flat on the animals back. They are mostly nocturnal scavengers in warm climates. Termites are highly social insects that are specialized for living on a diet of cellulose which is digested by a diverse array of micro-organisms in their guts. Other than the reproductives, termites are wingless. There are few species in the temperate zones of the world but they are abundant and ecologically important throughout the tropics.
Order Mantodea (Mantises). These are distinctive large predatory insects. Their front pair of walking legs is modified for grasping prey and are not used for walking. The forewings are leathery and folded as in cockroaches although the body is generally elongate rather than flattened. Their large size and striking appearance mean that they are familiar to most people although this is a small order with relatively few species. The large familiar species in North America are introduced from Asia. Most native species are smaller and less conspicuous.
Order Phasmida (Walkingsticks). Another small group of large distinctive insects. Like the mantids and the Blattodea this group is more abundant and diverse in the tropics. They are elongate, long-legged insects that feed on plants. Most species closely resemble twigs. North American species are generally wingless. This group includes some of the longest insect species, attaining lengths of over a foot.
Order Dermaptera (Earwigs). This group is only well known because some introduced species are common in human modified environments (i.e. yards and gardens). These are small to mid-sized insects with small wings and an elongate body. The forceps shaped cerci at the end of the abdomen are diagnostic.
Group 3 - Less conspicuous orders that are likely to be encountered These orders are ones that the average person is less likely to encounter/notice in their day to day lives. However they are insects that occur widely and should not be difficult to find in the appropriate environment if you look for them.
Order Microcoryphia (Bristletails) and Order Zygentoma (Silverfish). These two order are very similar and were formerly combined into one. These animals are ground-dwelling scavengers. A few species live in buildings but most are found in leaf litter, in soil, or under objects on the ground.
Order Ephemeroptera (Mayflies). This order and the following one have aquatic juveniles. The adults are not strong fliers and are seldom found a long way from water. Adult Mayflies do not feed and only live a few days. They often emerge in large numbers and have spectacular mating swarms. They are found in a wide variety of aquatic habitats with most families specializing in a particular type (e.g. fast flowing streams).
Order Plecoptera (Stoneflies). Have aquatic nymphs similar to those of the Ephemeroptera and Odonata but the adults are much less active and conspicuous. Adults may or may not feed and may be nocturnal or diurnal. Most diverse in streams.
Order Megaloptera (Dobsonflies, Alderflies). Have aquatic larvae that are predaceous. Larvae have a well developed head with eyes and mouthparts. They are elongate with gills on the abdomen and short but well developed legs. The adults are large insects with large membranous wings that are held rooflike over the body at rest. They are weak flyers and usually not found far from water.
Order Neuroptera. Similar to the Megaloptera in general body form but smaller and more varied in shape. Larvae are mostly active predators that can either be elongate (lacewings) or short and wide (antlions and owlflies).
Order Mecoptera. Scorpionflies. This is a small order of medium-sized insects. They are mostly found on vegetation and are generally predators or scavengers. The Mecoptera have an extensive fossil record and are thought to be one of the most ancient of the insect groups with larval and pupal stages.
Order Siphonaptera (Fleas) Very small insects that are flattened from side to side. Wingless, with long legs. Very powerful jumpers. External parasites of birds and mammals. Most humans are only likely to encounter dog and cat fleas but there are many other species with other hosts.
Order Trichoptera (Caddisflies). Larvae are caterpillar like and aquatic. Mostly live in tubelike structures made by cementing debris together. Adults resemble moths but have hairy wings and very long antennae. Mouth parts are chewing. Weak fliers and not often found far from water.
Group 4 - Orders that are unlikely to be encountered without a special effort These are either very small or occur in very specialized environments. Most of them are not in identification section above.
Order Embiidina - Webspinners.Tiny, elongate, mostly tropical insects. Females are cylindrical and wingless. Males are flattened and usually winged. Females and their offspring live in silken galleries in sheltered places. A small number of species occur in southern states. A substantial number of them lived on the uneven stucco surface of my house in Phoenix. They were so small that most people would never have known they were there.
Order Notoptera - Rock Crawlers. Long slender wingless insects, vaguely resembling an elongate cricket without the enlarged hindlegs. Discovered just under 100 years ago by Canadian entomologist E.M. Walker these insects are restricted to cold environments, usually at high elevations. This is probably the smallest insect order with only about 20 species found in western North America, Siberia, and Japan.
Order Zoraptera - Zorapterans. Extremely tiny, termite like insects. This may be the second smallest insect order with fewer than 50 known species worldwide. They are usually found in association with rotting wood.
Order Pscocodea - Lice. I had a hard time knowing where to place this order. It contains the parasitic chewing and sucking lice (all wingless) and the free living book lice. This is a larger and more widespread group than most of the other orders in this last section. However to my knowledge I have never seen a louse of any kind so I am placing them down here. These are all very small insects.
Order Thysanoptera - Thrips - Tiny plant feeding insects related to the Hemiptera. They are elongate with sucking mouthparts and wings fringed with hairs. They are very very small. Although not uncommon they are seldom noticed because of their tiny size.
Order Rhaphidioptera - Snakeflies. Listed in the identification section above. Related to the Neuroptera, an order in which they were once included. Predatory insects mostly found under tree bark. Small number of species restricted to the western states.
Order Strepsiptera - Twisted winged parasites. These are among the oddest of insects and their classification has long remained uncertain (sometime they are considered a suborder of the Coleoptera). They are minute parasites of other insects. Males are vaguely like a beetle that has lost its hardened forewings (elytra) - they have membranous hind wings and no fore wings. Adult females frequently lack eyes, antennae and legs and remain restricted to living in their parasite host.
Appendix: Variation in Classification
If you have an older field guide in particular you may come across a slightly different classification scheme. Below are listed some common variations.
Diplura, Collembola, and Protura listed as orders of insects rather than separate classes
Zygentoma and Microcoryphia combined into a single order, Thysanura
Phasmida, Cockroaches, and Mantodea all included within the Orthoptera
Termites given their own order, Isoptera
Hemiptera split into two orders: the Hemiptera (the suborder Heteroptera) and the Homoptera (suborders Auchenorryncha and Sternorryncha).
Psocodea split into either two (Psocoptera, Phthiraptera) or three (Psocoptera, Mallophaga, Anoplura) orders.
Megaloptera, Rhaphidioptera, and Neuroptera all combined into one order, the Neuroptera.