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 Siphunculata, 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. 



Siphunculata: metamorphosis incomplete; mouthparts fitted 

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

 wings secondarily lost. Human parasites: sucking lice. 



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. 



