INSECTS - SUNY Orange



INSECTS

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If numbers of individuals or numbers of species are used as criteria to measure success, insects are the most successful group of animals on earth. They are extremely important in food chains, in the pollination of flowers (including the pollination of crops), in scavenging dead organic matter, in their medical impact (as the vectors of a number of diseases) and in their economic impact (as agricultural pests and as the producers of silk, honey, shellac, and a number of other materials).

Most insects are small; three quarters of insects are under 6 mm in length. The smallest insects are only about a quarter of a millimeter and the largest ones have a wingspan of 30 cm (some fossil insects had wingspans of more than 75 cm) (Borror, 1981).

The insect integument is its skeleton, an exoskeleton. Muscles attach to the exoskeleton and the skeleton’s elasticity can oppose the action of a muscle the way an antagonistic muscle in vertebrates would. The exoskeleton is composed of a thin epidermis over a cuticle and the cuticle may compose up to half the dry weight of the insect. The main component of the insect exoskeleton is the protein chitin. Chitin is flexible and therefore other compounds perform hardening (or sclerotization) of this exoskeleton. While the exoskeleton of a caterpillar can be quite pliable while the jaws of certain beetles are so tough that they can bite through some metals (Romoser, 1981).

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The insect head possesses antennae, mouthparts, compound eyes, and simple eyes. It is thought that the eye resulted from the fusion of many primitive segments (estimated 4-7). The compound eyes are composed of hexagonal units called ommatidia. Some insects lack compound eyes, such as the larvae of some insects and some castes of social insects. Insect antennae are segmented and can occur in a variety of forms such as thin and tapering, clubbed, feathery, and similar to a string of beads. Some insect antennae are sexually dimorphic. Some insects use eardrums to hear (which may be located on their legs or on the abdomen) while others use sensitive hairs (Romoser, 1981; Borror, 1981).

Insect mouthparts are thought to have arisen from the appendages of the ancestral segments which fused and were modified. Although the mouthparts of many insects can be generally referred to as biting or sucking, there are a variety of modifications. For example, dobsonfly males have specialized mouthparts for holding females in mating. Mouthparts may be modified over the life of an insect. For example, the larvae of mosquitoes possess mouthparts which are adapted for filter feeding as opposed to the sucking mouthparts of adult females. Larval dragonflies possess extendible mouthparts and the mouthparts of butterflies form a long, extensible coil (Romoser, 1981; Borror, 1981).

The insect thorax is divided into 3 segments, all 3 of which possess a pair of legs in most insects. In winged insects, the middle and hind segments of the thorax possess a pair of wings. Insects are the only invertebrates which evolved wings. Openings of the respiratory system called spiracles open onto the thorax. Insect legs are usually composed of 6 segments. Legs can be modified for digging (usually forelegs), jumping (hindlegs), swimming, grasping prey. Larval insects different leg appendages (Romoser, 1981).

The insect abdomen is segmented. The number of primitive segments is thought to be 11, of which the final segment may be reduced and hold appendages known as cerci. Some cerci resemble pinchers, others are long sensory structures, and others (such as those of damselfly larvae) bear gills.

Unicellular cuticular processes known as setae can be modified to form hairs and scales. Dragonfly larvae can force water through their rectum to propel them forward. In ants, bees, and wasps, the egg-laying structures at the tip of the abdomen have been modified to form a sting. The stinger of ichneumon wasps may reach 10 cm and can enter wood (Romoser, 1981; Borror, 1981).

Insects have a central nervous system composed of a brain, a series of ganglia, and the nerve cords which connect them. The brain is capable of learning and secretes neurohormones. A number of glands exist whose secretions may aid in digestion, deterrence of predators, or intraspecific communication. Some insects possess glands which produce waxes. Stinging hymenopterans can produce a number of venoms in their glands. Glands in some caterpillars can produce poisons associated with hairs. Salivary glands can secrete silk in caterpillars. Some bloodsucking insects produce anticoagulins and some insect salivary glands produce glues to attach pupal cases to surrounding objects.

A number of insects have muscular pump around first part of digestive tract. The midgut is the primary site of digestion and absorption. While some insects have generalized diets, others more specialized (such as those that feed on wood or blood) and as a result, digestive enzymes vary among insects (Romoser, 1981).

Insects possess open circulatory systems in which a dorsal heart pumps fluid throughout body. This fluid serves the functions of both vertebrate blood and lymph (and is therefore referred to as hemolymph). The number of blood cells per cubic millimeter of hemolymph varies from 1,000 to 100,000 (Romoser, 1981; Borror, 1981).

Air is brought in and out of the insect body through openings called spiracles. It travels through tubes known as trachea, tracheoles, and, in many insects, air sacs. Aquatic insects may have a variety of gill structures extending from the body with a rich supply of trachea. They are located on the legs of stonefly larvae, on abdominal segments of mayfly larvae, inside the rectum in dragonflies, and on the tip of the abdomen (and in the rectum) in damselflies. Other aquatic insects breathe atmospheric air through spiracles in their abdomen which contacts the surface, a breathing tube from the tip of their abdomens, or spines inserted into the air spaces of plants. Some aquatic insects use a temporary store of air either inside or outside the body to dive for extended periods (Romoser, 1981; Borror, 1981).

Although most insects lay eggs, some may give birth to larvae or pupae. Most insects reproduce sexually, but there are parthenogenic species known which are composed only of females. In some insects a single embryo can divide to produce numerous offspring. In the genus Glossina, larvae can receive nourishment in the “uterus”. Many insects cover their eggs in protective cases or froths, others laid singly.

Wingless insects (apterygotes) do not undergo significant changes as they mature from larvae to adults. In winged insects, many are hemimetabolous in that their larvae resemble adults (although they lack wings and reproductive structures). Some aquatic insects, the larvae differ more. Some hemimetabolous insects form a pupa before becoming adults (such as thrips and some homopterans) and some aquatic insect larvae are structurally more different than adults. Holometabolous insects are those that undergo a complete metamorphosis in a pupal case. Butterflies (whose larvae are caterpillars) and flies (whose larvae are maggots) are examples of this type of life cycle. The larvae of holometabolous insects often lack compound eyes and thoracic legs and may possess abdominal legs (Romoser, 1981; Borror, 1981).

Some adults very short life such as 15 days in some mosquitoes, 5-10 days in a number of butterflies and moths, and a single day in some mayflies. Other insect adults may survive 2 years. Insects may produce a number of broods of offspring per year, allowing their populations to grow very quickly. For example, the progeny of one pair of fruit flies (if given enough food, space, etc) that could be produced in one year could from a sphere about the size of the earth’s distance from the sun (Romoser, 1981; Borror, 1981).

Insects can perceive a number of sensations and can possess receptors for taste, sound, vision (including movement, color, and distance), touch, vibration, heat, moisture. Bioluminescnence is known in springtails, homopterans, beetles, and true flies. Insects can produce sound through a variety of mechanisms: tapping on surfaces (in death watch beetles, some grasshoppers, and termites). Others produce sound through friction/vibration of their wings (lepidopterans, hemipterans) (Romoser, 1981).

Walking is similar in all insects. Caterpillars move through peristaltic waves, body extension and contraction. A number of insects jump including grasshoppers, fleas, hoppers, flea beetles, and springtails. The primitive condition for wings is wings which move separately as in dobsonflies and termites. These insects do not fly too well. In other insects, the wings move as a unit. Dragonflies can fly with 40 wingbeats per second, flies may reach 400 beats/second, and some midges can reach 1,000 beats/ second. Honeybees and horseflies can fly at 22 km/hr, hornets 25, the fastest dragonflies 25-30 km/hr, and deer bot flies can reach 40 km/hr (Romoser, 1981; Borror, 1981).

Some insects migrate, such as cotton leafworms and some leafhoppers. The migrations of locusts can involve huge quantities of individuals. These migrations in the Great Plains of the U.S. in the past have been estimated to contain 124 billion locusts, and spread 300 miles longs and 100 miles wide. Monarch butterflies may migrate 1500 miles southwards in winter and 1000 miles northward the following spring (Borror, 1981).

Bioluminescence is known in some springtails, gnats, and three families of beetles commonly called fireflies. Fireflies possess organs which contain a substance called luciferin which, upon nervous stimulation, is oxidized using the enzyme luciferase to produce light.

Ants, termites, and some bees and wasps may form complex societies composed of different classes of individuals (castes). Ant and bee colonies consist of queen(s), males called drones, and female workers. Ant societies may “cultivate” fungi upon which they feed or “tend” herds of aphids, feeding on their excreted sugars. Some workers may function as “soldiers”, often with greatly enlarged mandibles. Societies may excavate tunnels or build nests.

WINGLESS INSECTS

ORDER PROTURA

The insects of this order live in decaying organic material.

ORDER COLLEMBOLA

These wingless insects can exist in populations of millions. They can live in the arctic and can be found on the surface of snow in this region. They can be found on the surface of water and in decaying organic matter. There may be 100,000 of them in a cubic meter of soil and millions per acre. They possess a long furcula on their abdomen, which, when released, causes them to spring.

ORDER THYSANURA

This order includes the silverfish, which can exist in moist regions with humans in addition to forest litter. In the following images of thysanurans, two primitive conditions are evident: these insects lack wings, and they still possess additional abdominal legs, reminiscent of the condition of other arthropods.

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One of the extinct orders of wingless insects, Order Monura, resemble crustaceans in some ways but possess the insect characteristics of 1 pair of antennae and 3 pairs of walking legs on three thoracic segments (Sharov, 1966).

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Vestigial abdominal legs are known in fossils Carboniferous Monura and Thysanura, Carboniferous and recent Diplura, and nymphs of Paleozoic Paleoptora (Kukalova-Peck, 1987)

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WINGED INSECTS

An early major development of insect evolution was the evolution of wings. While the orders Thysanura, Archeognatha, and Monura (all Ectognatha) are considered wingless, there are thoracic side lobes which are composed in part of a protowing which fuses to the rest of the thorax (among the Monurans, only in extinct species). Modern members possess a pattern of blood lacunae and tracheae similar to the venation of insect wings. In fossil species of Monura and Thysanura from the Carboniferous, the protowings are not fused to the thorax and resemble the winglets of immature winged insects from the Paleozoic. There is evidence to suggest that the first insect wings were derived from respiratory structures on the legs of ancestral arthropods gathered from embryology (wings are similar to epipodites and develop from the same larval disc as legs and the wing pouch separates from the leg pouch), genetics (given a homeotic mutation that can cause the formation serial wings on the segments of the abdomen), anatomy (similarities in the musculature and location of wings and epipodites), neurophysiology (abdominal neurons suggest that wings were derived from serial structures which continued into the abdomen, as observed in the mobile gills of fossil mayfly larvae), and paleontology (the wing cases of fossil larvae weren’t fixed as in modern forms, but were mobile). The insect wing did not begin its evolution in insects, it was derived from leg appendages already present in the ancestors of insects. (Kukalova-Peck, 1987)

How did insect wings evolve? Although there is not enough evidence to reach a conclusive answer, two possibilities have some support:

1) Insect wings could have evolved from tergal lobes on the thorax. One of the early orders of insects, the Paleodictyoptera resemble mayflies and have, in addition to 2 pairs of fully developed wings, a pair of tergal lobes on the prothorax. It is possible that the ancestor of winged insects had three pairs of tergal lobes, of which the caudal two pairs developed into wings (Sharov, 1966; Callahan, 1972). Prothoracic lobes were present on some of the Protorthoptera (Carpenter, 1941)

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2) Insect wings could have evolved from epipodites in more primitive arthropods. Below is a crustacean limb with epipodites.

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Two genes which are important in the early development of wings (apterous and pdm) are also present in dorsal respiratory appendages of crustacean limbs (called epipodites). Different branches of crustacean legs show a pattern of expression of another important gene (engrailed), which is similar to the expression observed in insect wings and legs.

The most primitive groups of winged insects (including dragonflies, damselflies, mayflies) alive today can’t fold their wings over their bodies. The extinct order of insects Protodonata (some of whose members resemble dragonflies) included the largest insects in history, such as those which had a wingspan of 70 cm (Callahan, 1972). Protodonata were large insects and include the largest known insects with wingspans of 2.5 feet (Carpenter, 1953).

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