Phylum Arthropda: Subphylum Hexapoda

Dr. Elizabeth A. Bergey and Dr. Eric G. Bright, University of Oklahoma

Modified, with permission, from Invertebrate Anatomy OnLine
copyright 2003 by Richard Fox (Lander University)

Hexapoda Overview

The hexapod body is divided into three regions: head; thorax; and abdomen.  Appendages are uniramous and a single pair of antennae is present.  Three pairs of legs and two pairs of wings are found on the thorax of most adults.  Gas exchange is via trachea.  Excretory organs are Malpighian tubules and the end product of nitrogen metabolism is uric acid.  There is relatively little cephalization of the nervous system. Insects are dioecious with copulation and internal fertilization.

Entognathous Hexapods

Collembola, Protura, and Diplura, the sister taxons of Insecta, are primitive hexapods with mouthparts partly in a depression on the head (i.e., they have ‘cheeks’).  Wings are absent (they were never present).  Development is ametabolous.  Malpighian tubules and compound eyes are reduced. We will only cover Collembola in this course.

Class Collembola (springtails)

Collembolans (springtails) are small - rarely exceeding 5 mm in length.   The 7,000 described species are found in a variety of habitats including leaf litter, soil, single meter of forest floor may support as many as 60,000 individuals. 

Most springtails have a springing mechanism consisting of a spring (= furcula) on the forth abdominal segment and a catch mechanism on the third.  The spring is under tension.  When the catch is released, the spring ‘springs’ and the springtail can be propelled up to 20 cm.  Some species, especially thse living in soil, have lost the spring.

A short cylindrical process, the ventral tube or collophore, is found on the ventral midline.  The ventral tube functions in water uptake, and is well developed in dry-habitat species. 

Springtails exhibit a variety of feeding modes including herbivory, carnivory, and detritivory.  Fungi are important in the diets of many.  Others suck the juices from plants.  Some consume arthropod feces, pollen, decaying plant matter, or algae.
Development is ametabolous and molting continues after sexual maturity. 

An entomobryomorph collembolan

Three types of springtails differ in general morphology, external segmentation, jumping mechanisms, eyes, habitat, and other characteristics. Two are elongate springtails. One type (entomobryomorphs) lacks a pronotum, so looks like it has a neck. The legs and antennae are long, and the furcula and eyes are well developed. Entomobryomorphs are characteristic of forest leaf litter. The second type of linear collembolans (podomorphs) are more squat. The pronotum is present; hence they do not have a ‘neck’.  The legs and antennae are short, and the spring, ventral tube, and eyes are small or absent.  Podomorph species are typically found in soil and deep litter.  They are intolerant of desiccation.  The third type are the globular springtails. In these, the thoracic and abdominal segments are fused to produce a short, rounded body form.  External segmentation is not evident.  The antennae are well developed and a narrow “neck” is present between the head and thorax.  T these are active animals and the spring is well developed.  Globular springtails are characteristic of grassy vegetation and open habitats, and are more tolerant of desiccation than the two types of elongate springtails.

Collembolans are easily collected from forest leaf litter using a Berlese funnel.  A few handfuls of leaf litter are placed on a screen in a large funnel.  A light bulb is left on over the funnel. The heat from the light, and the resulting desiccation, drives the leaf litter animals downward in the funnel until they drop into a vial.  The Berlese technique will yield a variety of small soil arthropods, typically including many species of mites and collembolans as well as pseudoscorpions, spiders, millipedes, centipedes, and ants. 

Class Insecta

Most hexapods are insects.  Insects have ectognath (fully exposed) mouthparts and the adults of most have wings.

Order Ephemeroptera (mayflies)

Ephemeroptera means ‘temporary wings’, which refers to the short life span of the adults. Nymphs are aquatic and are characterized by 3 cerci (= ‘tails’) and gills down the sides of the abdomen – some of the gills may be lost or modified in some taxa. Nymphs are most commonly grazers of algae and other components of biofilms, but some are predators or filter-feeders. Most of the life cycle is spent as nymphs in freshwater.

Mayflies are hemimetabolous and the forewing pads are evident in more mature nymphs. At emergence, mayflies molt at the water surface and, when their wings are dry enough, adults fly to shore. These are subimagoes, with dull colors and ‘filmy’ wings. After a day or two, the subimagoes molt and become imagoes, which are shiny, relatively more colorful and have clear wings (where not pigmented). This is the only insect order that molts as winged adults. Adults live a few days.

Characteristics of adult mayflies are: (1) the forewings are much larger that the hindwings, (2) there are 2 ‘cerci’ (versus 3 in nymphs), and (3) the wings cannot be folded - sticking up when closed (as in a butterfly).

Adult mayflies do not feed and have non-functional mouthparts and gut. They reproduce and disperse (usually not travelling far from water).

Order Odonata (dragonflies and damselflies)

Like mayflies, odonates are hemimetabolous, have aquatic nymphs and terrestrial adults, and have wings that do not fold. Odonates are predators as both nymphs and adults, consuming mostly other invertebrates      - although large dragonfly nymphs may consume small fish. Odonates have chewing mouthparts. Nymphs have a hinged labium with a pair of moveable hooks at the end, which form a ‘mask’ when retracted. Many are lie-in-wait predators. The labium is thrust out rapidly to catch prey, which are held with the hooks. Adults catch flying insects in the air.

Damselflies and dragonflies differ morphologically in both the nymphal and adult stages.

Damselfly nymphs are relatively thin and most have narrow abdomens. The abdomen ends with 3 large gills. Gills are held apart when resting, but come together to form an expanded ‘tail’ when swimming (the side-to-side swimming movement resembles a swimming fish). In contrast, dragonfly nymphs are relatively stout,

Adult damselflies are thinner, more delicate-looking, and often smaller than adult dragonflies. Behaviorally, adult damselflies rest with their wings ‘down’, whereas adult dragonflies hold their wings horizontally at rest. In reality, damselflies hold their wings vertically, but the thoracic segments are so sharply angled that this vertical position is along the abdomen.

Order Orthoptera (grasshoppers, crickets, locusts, and katydids)

Orthoptera means ‘straight wings’, so-named because the wings extend their entire length (though they may have longitudinal pleats). Orthopterans are characterized by: (1) large hind legs adapted for jumping, (2) a large pronotum, and (3) thickened forewings. Females have a large thin ovipositor but male genitalia are not visible externally. Males stridulate (produce noise through rubbing) in many species. Immatures are nymphs (i.e., they are hemimetabolous).

Orthoptera includes about 20,000 species. Most are herbivores or detritivores.  

An external morphology exercise is given in a separate document.

Order Dermaptera (earwigs)

Dermaptera (‘skin wings’) refers to the thickened skin-like forewings. Earwigs are characterized by: (1) paired cerci (forceps) that are often pincher-like and (2) short thickened forewings. Earwigs may be detritivores, herbivores, or predators.

Earwigs are nocturnal. They are hemimetabouous orthopteroid order with maternal care of eggs and young earwigs. The name ‘earwigs’ may come from ‘earwing’ because the hind wings are somewhat shaped like human ears. Earwigs are not medically important (and don’t get in ears!!).

Order Mantodea (praying mantids)

Praying mantids are characterized by: (1) thickened forewings; (2) a very long pronotum; and (3) raptorial forewings. They are an hemimetabouous orthopteroid order. Praying mantids are lie-and-wait predators that often mimic or blend in with their environment. They eat both insect pests and beneficial insects.

Female praying mantids are well-known for the consumption of males, which may occur during the mating process. Consumption of males provides nutrient for egg development. Although widely reported, the occurrence of male consumption may be exaggerated because most studies are in cages, where males are less likely to escape.

Although there are native species of praying mantids, two introduced (the Chinese and the European praying mantids) are very common.

Order Blattodea (cockroaches, wood roaches, and termites)

Roaches are: (1) dorsoventrally flattened; (2) have a large pronotum with an overhang that obscures the head from above; and (3) have leathery forewings. Blattoidea is a hemimetabouous orthopteroid order. Eggs are usually in a protective egg case (ootheca); although some roaches retain the eggs and give birth to nymphs (ovovivipary).

Roaches are an ancient group that has changed little over millions of years. Their ability to hide and fit into crevices has certainly helped the group survive. Roaches are most diverse in the tropics.

Although most people dislike roaches, this attitude is based on pest species, which account for less that 1% of known species. People have very little contact with most species (e.g., wood roaches in Oklahoma) and roaches may even be kept as pets.

Termites are hemimetabolous eusocial insects. A nest of termites normally includes three morphologically different castes: the queen (the sole egg layer who can be quite large), workers, and soldiers. Winged male and female reproductives occur seasonally, and leave the nest to mate and start new nests. Queens can be long-lived – some termite mounds are at least 70 years old. The king is also long-lived and mates periodically.

Many termites are blind. Pheromones are commonly used in communication.

Termites feed on cellulose directly (e.g., wood) or indirectly (e.g., fungi) and have symbiotic protozoa and/or bacteria to digest cellulose. Many are associated with cycling of nutrients and aeration of soils. Some are structural pests, agricultural pests, or can weaken earthen dams.

Wings are of similar size and shape of the wings (which occur only in reproductives). Wings are soon lost.

Order Plecoptera

Stoneflies are aquatic (in streams and rivers = flowing waters) as nymphs and terrestrial as adults (though they occur close to water). Both adults and nymphs have 2 prominent cerci (‘tails’) and, in adults, the wings are held flat over the back. Nymphs are grazers or predators; adults don’t feed much. In locating mates, males of some species drum the substrate with their abdomens.

Stoneflies are aquatic (in streams and rivers = flowing waters) as nymphs and terrestrial as adults (though they occur close to water). Both adults and nymphs have 2 prominent cerci (‘tails’) and, in adults, the wings are held flat over the back. Nymphs are grazers or predators; adults don’t feed much. In locating mates, males of some species drum the substrate with their abdomens.

Stoneflies are aquatic (in streams and rivers = flowing waters) as nymphs and terrestrial as adults (though they occur close to water). Both adults and nymphs have 2 prominent cerci (‘tails’) and, in adults, the wings are held flat over the back. Nymphs are grazers or predators; adults don’t feed much. In locating mates, males of some species drum the substrate with their abdomens.

Order Thysanoptera (thrips)

Thrips are very small insects, so are seldom noticed – although they may be quite common. Thrips have sucking mouthparts and have one mandible (on the left side). Most feed on fungi, some are herbivores, and a few are predators (preying on other thrips and small arthropods). Wings of thrips are linear and fringed. A few thrips are pests, primarily as vectors of viral plant diseases.

Order Hemiptera (true bugs; cicadas; aphids; plant, leaf, and tree hoppers)

Hemipterans are a hemimetabolous order with sucking mouthparts. Most hemipterans are plant feeders but some are predators, and a few are parasites (e.g., bedbugs).

There are two groups of hemipterans that differ in how the wings are held at rest. In one group, the wings are held flat over the abdomen and the forewings have two sections: a thicker basal part and a distal membranous part that overlaps with the wing from the other side. In the other group, the wings have the same texture throughout and are held ‘tent-like’ over the back.

Order Phthiraptera (chewing and sucking lice)

Lice are all ectoparasites of vertebrates. Lice are characterized by: (1) well-developed claws for grasping and (2) a lack of wings (secondary loss). Because lice are hemimetabolous and lack wings, lice appear (but are not) ametabolous, in that immatures look like miniature adults. Chewing lice have wider heads to accommodate well developed mandibles and musculature; sucking lice have narrower heads. Head and pubic lice are both sucking lice.

Order Coleoptera (beetles, grubs)

Beetles comprise about 25% of all named plants and animals! There is a wide diversity of morphology and other traits. However, beetles are characterized by: (1) protective, often hardened, forewings (= elytra) that meet at the midline and that cover folded membranous hindwings and the top of the abdomen; (2) a distinct and separate prothorax; and (3) chewing mouthparts.

Although beetles eat a wide range of foods, very few are parasitic.

Beetle larvae have a head capsule, 3 pairs of legs, and a long abdomen. Some are called grubs.

Order Neuroptera (lacewings, owlflies, and ant lions)

Neuroptera mean ‘nerve wings’, which is descriptive of the numerous veins in the wings. In particular, the series of short veins running perpendicular to the front edge of the wings is diagnostic for the group. Wings are held tent-like over the body.

Both adults and larvae are predators of soft-bodied invertebrates.  Adult green lacewings are known for eating aphids. Larvae have large mandibles and are lie-in-wait predators – catching ants and other small insects that fall in their sand traps (antlions) or wandering about in the forest floor (owlflies).

A related order, the Megaloptera, are aquatic as immatures and share the characteristic parallel veins on the front of the wing.

Order Hymenoptera (ants, wasps, and bees)

Hymenoptera mean ‘membrane wings’, which is apt. Characteristics include: the forewings are larger than the hindwings and the two wings are linked by hooks during flight, mouthparts are chewing (although they may be modified for sucking in some bees), and some are social, living in small to large colonies.

Adult ants have one or two small segments at the front of the abdomen (the pedicel), which forms a constriction between the thorax and abdomen. Although some wasps also have a constriction, they lack a pedicel. Bees are characterized by branched hairs.

Hymenopteran larvae are diverse, ranging from caterpillar-like (with legs and prolegs) to maggot-like (legs and prolegs lacking).

Hymenoptera consume a wide array of foods. They are important pollinators, parasitoids, and predators; some are herbivores or form galls.

Order Trichoptera (caddisflies)

Trichoptera means ‘hair wing’, a reference to the hairs that are on their wings. They are closely related to the Lepidoptera, whose wings have scales rather than hairs. Adults resemble moths and hold their wings ‘tent-like’ over their bodies. Antennae are long and filiform. Adult caddisflies are terrestrial and occur near water. Some do not feed; others are longer-lived and can consume liquids (water and nectar). Mouthparts are chewing (versus sucking in Lepidoptera). Mates are located using pheromones.

Caddisfly larvae resemble caterpillars but are aquatic in freshwaters. They produce silk from modified salivary glands and this silk is used for building tube-shaped cases, nets for filter feeding, and retreats. Larvae are algal grazers, detritivores, predators or filter feeders. Pupation occurs underwater.

Order Lepidoptera (butterflies and moths)

Lepidopteran adults are characterized by (1) large, sometimes colorful wings, (2) scales on the wings and body, and (3) sucking mouthparts that are coiled. Butterflies hold their wings up at rest and have clubbed antennae; moths hold their wings ‘tent-like’ or flat outward and have filiform or plumose antennae. In general, butterflies are diurnal and moths are nocturnal.

Larva are caterpillars and are herbivores with (1) a well-developed head capsule with chewing mouthparts, (2) thoracic legs and abdominal prolegs, and (3) salivary glands modified as silk glands. Hooks on the prolegs are called crochets. Pupae of this holometabolous order may be in a silken cocoon or may be without a cocoon (called a chrysalis).

External anatomy of a generalized caterpillar.

Order Siphonaptera (fleas)

Fleas are small, secondarily wingless, laterally compressed blood sucking insects. Most fleas are mammalian ectoparasites but a few species parasitize birds. About 2400 living species are known. The mouthparts are adapted for piercing.

Development is holometabolous and the larvae are slender and wormlike. Larvae are not parasitic and feed on organic detritus.

Fleas are adapted for moving through the hair or feathers of the host. Adaptations for this are the smooth, compressed body, absence of wings, posteriorly pointing spines and setae, and short antennae that can fit into grooves on the side of the head. 

The cat flea.

Order Diptera (flies)

Flies are a large and diverse group. Diptera means ‘two wings’, so named because only the front pair of wings are wing-like. The hind wings are present, but are modified to form drum-stick shaped halteres. Halteres are used for balance during flight and help make flies especially agile fliers.

Adults have chewing or sucking mouthparts and feeding types include fungus-feeders, herbivores, predators and parasoitoids, blood-feeding, and liquid feeding (e.g., nectar).

Larvae are generally referred to as maggots, but there is much variation in morphology and the term is not applicable for all groups. Dipteran larvae lack segmented legs, though many have prolegs.

© Copyright by Elizabeth Bergey and Eric Bright 2016

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