10 MEDICAL ENTOMOLOGY 



parts of the external genital organs are the representatives of true 

 abdominal appendages. At a very early stage in the evolution of insects 

 each metamere is believed to have been a replica of its neighbour, and 

 this condition of affairs is found to a certain extent in the early stages of 

 the present forms ; but in the adult the primitive arrangement of the seg- 

 ments is obscured, and can only be recognized in certain parts of the 

 animal. The most obvious segmental markings are to be found in the 

 exo-skeleton, and particularly in the abdomen, and the term ' segment ' 

 has come to be used with a special meaning in this connection. In the 

 head there is little trace of the original arrangement to be found, and 

 the extent of the various segments is a debatable matter. In the thorax 

 each segment bears a pair of legs, but on account of secondary changes 

 brought about by the growth of the wings the segments have developed 

 unequally. In the internal anatomy the only .organs which show traces 

 of a segmental arrangement are the tracheae or breathing tubes, of which 

 there are as a rule one pair to each segment, and the ventral chain of 

 nerve ganglia. 



The Diptera are the most highly specialized as well as one of the most 

 numerous orders of the class, and in them we find that the fusion of the 

 segments has gone further than in other orders. As a result of the great 

 development of one pair of wings, with the muscles which act upon 

 them, the middle segment of the thorax has become greatly increased in 

 extent at the expense of the others. In the abdomen there is, through- 

 out the order from the simple forms upwards, a general tendency to a 

 reduction in the number of the visible segments, and a corresponding 

 concentration of the internal structures. This is frequently accompanied 

 by a transformation of the exo-skeleton of the terminal segments into 

 organs connected with the function of reproduction. 



Insects, like other arthropods, possess a more or less rigid integument, 

 which encloses and protects the soft parts, and at the same time pro- 

 vides attachments for the muscles of the body, all of 

 The exo-skeleton , . , , ,,. . . , . . . . ^ , 



and its appendages wnicri have their origins and insertions on the internal 



surface of the exo-skeleton. The appendages consist 

 of outgrowths of the integument, containing muscles, nerves, and other 

 structures, and the joints in them are formed by a moulding of the outer 

 layer. There is no true internal skeleton, but the exo-skeleton sends in- 

 wards certain processes, often termed the endo-skeleton ; these occur in 

 situations where either greater structural stability or additional surface 

 for muscle attachment is required. 



The nature of this integument is of considerable importance from 



