CHAPTER II 

 The Class Insecta, and the Order Diptera 



Class Insecta. 



It is not germane to the purpose of this work to attempt to 

 discuss the importance of insects either in the economy 

 of Nature at large or in their manifold bearings upon human 

 affairs ; it is sufficient to say that insects outnumber all 

 other species of animals put together, that it is impossible 

 to overestimate their natural influence and momentum, and 

 that so far as their relation to civilised man is concerned, 

 insects have in various wa.ys retarded his most resolute 

 efforts to replenish the earth, and have at times and in 

 places disputed his very dominion. 



Among Arthropoda the typical adult insect is distinguished 

 by the form of the body, the number and disposition of the 

 appendages, and the possession of wings. 



The body consists of three well-defined regions or 

 syntagmata — head, thorax, and abdomen. 



The head is a chitinous capsule, sharply differentiated 

 from the thorax ; except as indicated by the paired appen- 

 dages, it shows, as usual, no tr^ce of segmentation ; nor 

 does the number of paired appendages reveal its true 

 segmental composition. The thorax is composed of three 

 segments — the prothorax, mesothorax, and metathorax. 

 The prothorax is often distinct, as also, in insects that do 

 not fly much, are the other two segments ; but in insects 

 whose ordinary method of locomotion is flight, the meso- 

 thorax and metathorax at least are fused together to increase 

 the leverage of the wings. In the abdomen, which is 

 generally well demarcated from the thorax, the individual 



