THE THORACIC EXO-SKELETON. 165 



homology was Latreille, who regarded the halteres as abdo- 

 minal appendages. I shall show hereafter that Latreille was 

 clearly misled by the attempt to prove that the first abdominal 

 segment enters into the composition of the thorax in the 

 Diptera. 



The posterior margin of the wing is usually prolonged, 

 in all the Diptera with a heavy abdomen, in the form of two 

 semicircular scales, the anterior of which is the squamula, 

 or lesser wing scale, and the posterior the squama, or great 

 wing scale. Those Diptera which possess wing scales are 

 termed calypterate. 



Modifications of the Thorax. Graber says, in speaking of the 

 thorax in insects generally : ' It is especially an organ of 

 locomotion ; in the more terrestrial forms the three thoracic 

 segments are mobile on each other ; in aerial insects, on the 

 other hand, they are consolidated and form a rigid case, in 

 which the segment bearing the largest pair of wings is most 

 developed. The sternal region is chiefly developed in relation 

 to the legs, and when the three pairs are equal in size, the 

 three sterna are also more or less equally developed, whilst the 

 pleura and terga are largest in the wing-bearing segments. 

 Whenever the wings are reduced, there is a corresponding 

 reduction of the dorsal arch ' [10, vol. i, p. 85]. 



In illustration of the above statements, I would observe that 

 in the Coleoptera, in which the mesothoracic wings are reduced 

 to wing covers, or elytra, which serve as mere protective 

 sheaths, the dorsal arch of the mesothorax is much reduced. 

 In the Diptera, on the other hand, it is the metathorax 

 which has a rudimentary dorsum, and the whole segment is 

 greatly reduced. The coleopterous metathorax resembles 

 the dipterous mesothorax, and may be compared with it, 

 almost every sclerite of the one occurring in the other. 



In all aerial insects the prothorax is also greatly reduced in 

 size ; thus, in the Lepidoptera and Hymenoptera the dorsal 

 arch forms a mere collar (collar c). It is also separated from 

 the sternal portion by a syndesmosis, a fact which led Kirby 

 and Spence to deny the prothoracic origin of the collare. 



ii 2 



