378 NATURAL ARRANGEMENT OF INSECTS. 



hues. Amongst these, the remarkable genus Achias 

 is distinguished by the lengthy dilatation of the sides 

 of the head; at the extremity of which the eyes are 

 placed. Stomoxys and Hcematobia feed upon the blood 

 of animals, and the majority resort to animal secretions 

 or excretions ; but it is the females only which are 

 thus annoying. AU substances, vegetable or animal, in a 

 state of decomposition, afford pabulum for their young ; 

 and notwithstanding the celerity of these processes, their 

 developement keeps pace with it, and these larvae acquire 

 maturity by the time their provender disappears. 



(339.) The second section contains the An thorny via t 

 which are closely related in structural characters to the 

 former, and from which it is difficult to separate them 

 strictly. They are, however, less vigorous; and the 

 order here visibly commences decreasing in organisation : 

 they form a large group, which are frequently difficult 

 to determine specifically. 



(340.) The third section, the Acalypteridce, exhibits 

 seventeen sub-tribes : our space will not allow us to 

 notice more than the most conspicuous of them, for 

 they comprise the largest group throughout the Zh*- 

 ptera ; and which exhibits a progressive declension in 

 structure and organisation, displayed chiefly in their an- 

 tennae and wings; in some of the superior sub-tribes, 

 the former still exhibit considerable developement, 

 which gradually decreases. Some of the most remark- 

 able structural peculiarities are shown in the exserted 

 ovipositor and gaily marked wings of the Tephritidce; the 

 produced scutellurn of Celyphus, nearly covering the ab- 

 domen ; the laterally dilated head in Diopsis ; the en- 

 largement of the anterior femora in Ochthera, thence 

 named Mantis; and the singular and constant vibration 

 of the wings in the Sepsidoz and Ortalidcs. The order 

 here loses the active habits of the preceding divisions, 

 these insects being usually rather inert, and frequenting 

 the shades of woods and plants, and but rarely visiting 

 flowers and the sunshine. Some, as in the preceding 

 tribes, feed upon decaying animal and vegetable sub- 



