Insect

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Insects Fossil range: Early Devonian[1] (but see text) - Recent
Western honey bee (Order Hymenoptera)
Scientific classification
Kingdom: Animalia   Phylum: Arthropoda   Subphylum: Hexapoda   Class: Insecta Linnaeus, 1758

Insect anatomy
A- Head   B- Thorax   C- Abdomen
 

1. antenna
2. ocelli (lower)
3. ocelli (upper)
4. compound eye
5. brain (cerebral ganglia)
6. prothorax
7. dorsal blood vessel
8. tracheal tubes (trunk with spiracle)
9. mesothorax
10. metathorax
11. forewing
12. hindwing
13. mid-gut (stomach)
14. dorsal blood vessel ("aorta")
15. ovary
16. hind-gut (intestine, rectum & anus)
17. anus
18. oviduct
19. nerve chord (abdominal ganglia)
20. Malpighian tubes
21. tarsal pads
22. claws
23. tarsus
24. tibia
25. femur
26. trochanter
27. fore-gut (crop, gizzard)
28. thoracic ganglion
29. coxa
30. salivary gland
31. subesophageal ganglion
32. mouthparts
 

Insects (Class Insecta) are a major group of arthropods and the most diverse group of animals on the Earth, with over a million described species—more than half of all known living organisms^[2][3]—with estimates of undescribed species as high as 30 million, thus potentially representing over 90% of the differing life forms on the planet.^[4] Insects may be found in nearly all environments on the planet, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.

There are approximately 2,000 praying mantis, 5,000 dragonfly species, 20,000 grasshopper, 82,000 true bug, 120,000 fly, 110,000 bee, wasp and ant, 170,000 butterfly and moth, and 360,000 beetle species described to date. Estimates of the total number of current species, including those not yet known to science, range from two million to fifty million, with newer studies favouring a lower figure of about six to ten million.^[2][5][6] Adult modern insects range in size from a 0.139 mm (0.00547 in) fairyfly (Dicopomorpha echmepterygis) to a 56.7 centimetres (22.3 in) long stick insect (Phobaeticus chani).^[7] The heaviest documented insect was a Giant Weta of 70 g (2½ oz), but other possible candidates include the Goliath beetles Goliathus goliatus, Goliathus regius and Cerambycid beetles such as Titanus giganteus, though no one is certain which is truly the heaviest.^[8]

The study of insects (from Latin insectus, meaning "cut into sections") is called entomology, from the Greek εντομον, also meaning "cut into sections".^[9]

Body structure

Insects possess segmented bodies supported by an exoskeleton, a hard outer covering made mostly of chitin. The segments of the body are organized into three distinctive but interconnected units, or tagmata; a head, a thorax, and an abdomen. The head supports a pair of sensory antennae, a pair of compound eyes, one to three simple eyes ("ocelli") and three sets of variously modified appendages that form the mouthparts. The thorax has six segmented legs (one pair each for the prothorax, mesothorax and the metathorax segments making up the thorax) and two or four wings (if present in the species). The abdomen (made up of eleven segments some of which may be reduced or fused) has most of the digestive, respiratory, excretory and reproductive internal structures.

Nervous system

Their nervous system can be divided into a brain and a ventral nerve cord. The head capsule (made up of six fused segments) has six pairs of ganglia. The first three pairs are fused into the brain, while the three following pairs are fused into a structure called the subesophageal ganglion.

The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction. Some cockroaches have just six ganglia in the abdomen, whereas the wasp Vespa crabro has only two in the thorax and three in the abdomen. And some, like the house fly Musca domestica, have all the body ganglia fused into a single large thoracic ganglion.

Until very recently, no one had ever documented the presence of nociceptors (the cells that detect and transmit sensations of pain) in insects (e.g., ^[10]), though recent findings of nociception in larval fruit flies challenges this^[11] and raises the possibility that some insects may be capable of feeling pain.

Respiration and circulation

Insect respiration is accomplished without lungs, using a system of internal tubes and sacs through which gases either diffuse or are actively pumped, delivering oxygen directly to the adjoining body tissues (see Invertebrate trachea). Since oxygen is delivered directly, the circulatory system is not used to carry oxygen, and is therefore greatly reduced; it has no closed vessels (i.e., no veins or arteries), consisting of little more than a single, perforated dorsal tube which pulses peristaltically, and in doing so helps circulate the hemolymph inside the body cavity. Air is taken in through spiracles, openings on the sides of the abdomen. There are many different patterns of gas exchange demonstrated by different groups of insects. Gas exchange patterns in insects can range from continuous, diffusive ventilation, to discontinuous gas exchange.

References

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