2 ON THE ANATOMY OF THE FLY. 



it belongs, and to investigate its relations to other insects, not 

 only in its perfect state, -but in its various stages of development. 



Insects are articulate animals, organisms in which the ex- 

 ternal integuments fulfil the same purposes in the economy 

 as the osseous skeleton does in the vertebrata ; and, just as 

 the typical structure of the latter is most fully exemplified in 

 the skeleton, so the integumental system in the articulata affords 

 the most obvious indications of the type upon which they are 

 organised. 



In the lowest members of this great division of the 

 Animal kingdom we find the body composed of a series of rings 

 or Annuli, united to each other by a membrane, each ring 

 having a separate nerve centre, and being similar to every 

 other ring in the body of the animal, except the first, 

 which is modified so as to form a distinct head very low in 

 the scale of life. 



As we ascend, we find organs of sense developed upon the 

 anterior rings, however, still in a very rudimentary form; 

 an alimentary canal traverses the whole series, and a decided con- 

 centration of the nervous system takes place about its anterior 

 extremity. If we add to these characters the presence of a 

 mouth furnished with a pair of hooks, which not only serve the 

 purpose of teeth, but also assist the animal materially in locomo- 

 tion, and are in fact, one of three pair of modified limbs which 



from the anterior spiracle; it passes forward into the head and com- 

 municates behind with the lateral trachea. Two small pulmonary sacs 

 are also seen communicating in front with this trunk. The muscles of th c 

 deep layer are probably all more, or less concerned in flight. The ab- 

 domen is occupied in the mesial line by the dorsal vessel, on each side of 

 which are the large abdominal pulmonary sacs; behind these the con. 

 volated bile tubes are colored red. The salirary glands may be seen 

 crossing the pulmonary sacs. The sides of the abdomen are filled with the 

 mature ovaries, and its apex is occupied by the rectum and rectal papillae. 



