228 PTILOTA ! IMAGO EXTERNAL ANATOMY. 



segment itself is composed of a certain number of different 

 pieces, more or less distinct, with which it is essential to 

 possess an acquaintance. To arrive at this result, it is evi- 

 dent that we must examine one of the thoracic segments in 

 preference to the others, because they are most fully de- 

 veloped, and most complicated in their structure. The pieces 

 composing one of these segments are divided by M. Audouin, 

 who has devoted a great deal of attention to this branch of the 

 subject, into three distinct kinds dorsal, lateral, and ventral. 

 Mr. MacLeay, however, considers them only as dorsal and 

 ventral, uniting the lateral with the ventral. The ventral 

 portion consists of a simple piece, of which the form is very 

 varied : this is named the sternum. Attached internally to 

 this piece, at its posterior margin, is another single piece, 

 variable in its form, but generally resembling the letter Y, 

 and appearing necessary for the support and protection of 

 the nervous system. This piece, as being attached to the 

 head, thorax, or abdomen, is respectively termed the ento- 

 cephalus, entothorax, and entogaster. The lateral pieces are 

 more complicated, those on each side consisting of two ; the 

 one (anterior) articulating with the sternum, and directed 

 upwards, constituting the episternum, behind which the other 

 is situated, and which is in connexion with the base of the 

 leg, and is called the epimeron. The upper surface of the 

 thoracic segment is also composed of a number of distinct 

 pieces, which are respectively named pracscutum, scutum, 

 scutellum, and postscutellum, and which successively follow 

 each other. Such is supposed, theoretically, to be the struc- 

 ture of each segment of the body of the insect; and the 

 fixed number of parts thus discovered is supposed to exist in 

 all annulose animals, sometimes being distinct, as when the 

 segment has reached its maximum of developement, but still 

 more frequently more or less rudi mental, or soldered to ad- 

 joining pieces, as required by the variation of developemeut. 



