330 INTRODUCTION TO ARTHROPODS 



is the paradoxical fact that ideal environmental conditions 

 shorten the life of these insects, since they facilitate the early- 

 deposition of the eggs. 



Classification. — The classification of insects is based mainly 

 on three characteristics: the type of development, the modifi- 

 cation of the mouthparts, and the number, texture and venation 

 of the wings. All blood-sucking insects have mouthparts 

 adapted in some way for piercing and sucking, but the types 

 vary greatly in different groups. Many of the more thor- 

 oughly parasitic insects, e.g., lice, bedbugs and "sheep ticks," 

 have secondarily lost their wings entirely, or have them in a rudi- 

 mentary condition. In the whole order of Diptera the second 

 pair of wings is reduced to inconspicuous club-shaped append- 

 ages known as halteres. 



The only orders of insects which contain species of interest as 

 human parasites are the Hemiptera (Rhynchota), or true bugs; 

 the Anoplura, or sucking lice; the Siphonaptera, or fleas; and 

 the Diptera, or two-winged flies. These four orders may be 

 briefly summarized as follows: 



Hemiptera (suborder Heteroptera) : metamorphosis incom- 

 plete; mouthparts fitted for piercing and sucking, the piercing 

 organs being ensheathed in the jointed lower lip; first pair of 

 wings, unless reduced, leathery at base and membranous at tip; 

 second pair of wings, when present, membranous with relatively 

 few veins. Human parasites : bedbugs, cone-noses, kissing bugs. 



Anoplura: metamorphosis incomplete; mouthparts fitted for 

 piercing and sucking, and retractile into a pouch under pharynx; 

 wings secondarily lost. Human parasites: sucking Uce. 



Siphonaptera: metamorphosis complete; mouthparts fitted 

 for piercing and sucking, the piercing organs being ensheathed 

 in the labial palpi, and the mandibles modified as protective 

 flaps; wings secondarily lost. Human parasites: fleas, chiggers. 



Diptera: metamorphosis complete; mouthparts fitted for pierc- 

 ing and sucking, for sucking alone, or rudimentary; first pair 

 of wings (absent in a few species) membranous with few veins; 

 second pair of wings represented only by a pair of clubshaped 

 organs, the halteres. Human parasites: Sandflies, mosquitoes, 

 midges, blackflies, gadflies, tsetse flies, stable-flies, maggots of 

 various species. 



