HEAD OP WINGED INSECTS. 229 



Some pieces may even entirely disappear, and the segment 

 may appear to consist rather of a single piece, or, on the 

 contrary, of two or even a greater number of segments 

 united together. Modifications of this kind are traceable, 

 not only in different insects, and in the different segments of 

 the same insects, but even in any single determinate seg- 

 ment, when examined in the various states of the same indi- 

 vidual. The comparative exammation of these segments of 

 the insect skeleton has singularly simplified the anatomical 

 study of these animals, placing it upon a firm basis, and more 

 rigorously compelling the precise determination of organs 

 which had previously been either entirely overlooked, or had 

 only been regarded without reference to the analogous forma- 

 tions of other insects ; and hence to any organ which hap- 

 pened to be singularly developed, a name was given without 

 the least reference to the comparative structure of other 

 insects, or the same comparative parts in the other segments 

 of the same individual. 



It would carry us too widely into the field of theoretical 

 inquiry, were I to show that the head, and various abdo- 

 minal segments, are organized in a manner strictly analo- 

 gous to those of the thorax; neither could this be done 

 effectually without a reference to the tyj)ical structm-e of 

 annulose animals in general — a vast and difficult subject of 

 investigation. We will, therefore, now confine our atten- 

 tion to 



A. — The Head, or the Seat of the Organs of Sensation. 



The head, or first segment of the body, is united to 

 the anterior part of the thorax by a distinct articulation, 

 and may be considered as a kind of corneous case or box, 

 having an opening in the centre of its anterior part, where 

 the mouth is placed. It is of a triangular or oval shape, 

 with the narrowed part advanced in front ; the middle of the 



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