4 ON THE ANATOMY OF THE FLY. 



condition of this segment, and the extreme modification it under- 

 goes in the development of the perfect insect^ The fourth and 

 fifth segments are united above in the larva as well as in the 

 perfect form, and each bear a pair of sense organs ; the 

 antenna and the eyes. The sixth segment of the larva bears 

 the anterior spiracles or breathing pores ; but, together with the 

 next two, the seventh and eighth, and in fact all the succeeding 

 ones, is unprovided with legs. These three segments correspond 

 to the thorax of the perfect insect, which is however not only 

 provided with six legs, but also with a pair of wings, appendages 

 of the seventh segment. The eighth or last thoracic segment is 

 likewise usually supplied with wings in insects, but in the Dip- 

 tera, the class to which the fly belongs, they are suppressed and 

 their place is occupied by a pair of small organs, called halteres 

 or balancers. Of the remaining nine segments which form the 

 abdomen of the perfect insect, four are modified and converted 

 into the hard parts of the sexual organs, which are more or less 

 internal. 



Although the segments of the larva and perfect insect cor- 

 respond exactly in the manner above indicated, nevertheless the 

 head and thorax of the fly, from the fourth to the eighth seg- 

 ment inclusive, are not developed from the corresponding larval 

 segments. 



It may be here observed that the histological elements, that 

 is, the elementary tissues of the perfect fly, cells, fibres, &c., 



segments, or according to some, of six, making eighteen in the whole body; 

 but as the segments of the head of the fly, both in the larva and perfect 

 insect are very distinct, I am inclined to the former view, The thorax 

 constantly bears three pairs of legs and consists of three distinct segments ; 

 unless, as may appear probable from the position of the legs and spiracles, 

 the thorax has six, when the whole number of segments in the insect 

 would amount to twenty, the number found in the highest scolopendrife 

 (Scutigera. Lam. ), where the head consists of five segments, and the 

 body of fifteen, which each bear a pair of legs, but are united above, so 

 that only eight appear on the dorsum, 



