What insects are, and how they are classified 



bats. Many of these parasitic flies are wingless, as are a number of other 

 flies that Hve in exposed places or in caves. 



Bloodsucking flies are of great importance in the spread of disease (see 

 Chapter VIII). 



The larva of a fly is usually a grub or a maggot. Some have a distinct 

 head, but none has true segmented thoracic legs : a few have fleshy pro- 

 legs on the abdomen, but more often these are just patches of spines, 

 giving the larva an unshaven grubby appearance. 



The larvae of many flies live in water (e.g. mosquitoes (Figs. 45, 46); 

 midges) but most live in decaying animal or vegetable matter, and the 

 larvae of flies are one of the most powerful agencies in breaking down and 

 disposing of organic refuse. 



27. Siphonaptera (North America 250 j world iioo) 



Fleas (Figs. 55, 56). Wingless insects, flattened from side to side, and 

 with powerful legs with which they can jump vigorously. They are small — 

 the mole flea at 5 mm. (1/5 in.) long is a giant — shining brown, and 

 covered with spines and hairs, which project backwards, thus enabling 

 the flea to squeeze itself forwards among the hairs or feathers of its host. 



Adult fleas hve entirely by sucking blood from mammals and birds. 

 Larval fleas do not live on the host animal, but in cracks round about 

 its home, where they can feed on organic fragments from the host's skin, 

 and on the droppings of the adult fleas, which contain undigested blood. 

 In consequence of this habit, fleas are essentially parasites of animals that 

 have a den, a hole or a nest, to which they return frequently. Few fleas 

 are entirely confined to one host, and they will generally breed in associa- 

 tion with other hosts that have similar living habits : thus the ' hen flea ' 

 is abundant in the nests of a number of garden birds, as well as in hen- 

 houses. Furthermore, adult fleas will bite and take blood from hosts on 

 which they find themselves by chance, even if these are not their breeding- 

 hosts. 



When fleas are ready to emerge as adults from the pupa they have the 

 peculiar habit of lying dormant until they are disturbed. Thus when 

 people move into a house that has been empty for some time, they are 

 often alarmed that fleas appear suddenly. This habit is clearly an advan- 

 tage to a parasitic insect, because if it should emerge when there is no 

 suitable host nearby, the adult flea may well starve before it can find food. 



28. Hymenoptera (North America 14,600; world 105,000) 



BeeS:, wasps, ants and allied insects (Figs. 57, 59-63). These are recog- 

 nised by their membranous wings (from which they get their scientific 

 name), the veins of which form a network of cells as in Fig. 57; though in 

 many Hymenoptera, especially the small, parasitic ones, these veins have 

 been reduced to vanishing point. 



The Order is sharply divided into the sazvflies (Symphyta) and the rest 

 (Apocrita). 



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