INTRODUCTORY NOTE ON THE IIOMOLOGIES 

 OF INSECTS. 



In the following pages, I am aware, that in describing the nn- 

 tennal and optic segments as posterior to tbose composing the 

 mouth, I have done some violence to the ordinarily received 

 views of many distinguished naturalists ; but I think the sequence 

 of the cephalic segments in the fly is too plain to be easily mis- 

 construed. 



Professor Huxley,* states that he believes the upper surface 

 of the head in insects is sternal instead of dorsal, and he founds 

 this belief on the apparent curvature of the embryo ; but I think 

 it possible that the extraordinary development of the lateral parts 

 of segments behind the mouth, arching over and surrounding the 

 mouth segments, may give rise to the appearance 6f curvature. 



My knowledge of the Crustacea is too limited to allow me to 

 give a positive opinion, as to which segment is most anterior in 

 them, but I see no reason why the optic and the antennal seg- 

 ments should not be dorsal plates of segments posterior to the 

 mouth, largely deA'elopecl and arching over it, whilst the corres- 

 ponding ventral plates are either wanting or amalgamated with 

 the ventral portions of other segments. Such a supposition ap- 

 pears to me to be as likely to be correct as that their dorsal plates 

 are absent, whilst the ventral plates occupy the dorsal region, and 

 unite by their anterior border (made posterior by their flexion) 

 with the anterior edge of a dorsal" plate belonging to another 

 segment. 



As the names I have given to the various plates, forming the 

 head of the fly, depend upon their relations with each other, and 

 are mainly borne out by the distinctness of the four plates in 

 each segment, and the manner in which they articulate with each 



The agamic reproduction of Aphis, Linn. Soc. trans, vol. xxii. 



