hosford: head segmentation of insects. 69 



12. (OS.) The Ocular Sclerite. "Each compound eye is 

 situated in the axis of an annular sclerite which may be the 

 basal segment of the ocular appendages." These annular 

 sclerites are the ocular sclerites. They show especially well in 

 the cricket and Amblychila. 



13. (AS.) The Antennal Sclerites, annular sclerites at the 

 base of each antenna. 



14. (Tr.) The Trochantin of the Mandible, a distinct 

 sclerite between the mandible and the gense in some insects; 

 indeed, in all that I studied. 



II. The Structure of a Typical Segment. 



Since the abdominal segments have lost their appendages, 

 it is evident that a reduction of certain parts has taken place 

 there, which makes it impossible to use them as typical seg- 

 ments. The thoracic segments are the more typical. 



The parts of a thoracic segment as generally recognized are 

 as follows: a ventral part, sternite; two lateral parts, pleu- 

 rites; and a dorsal part, tergite. Each segment is composed 

 of two subsegments. "The line separating the subsegments 

 passes, on the pleural aspect, between the episternum and the 

 epimeron ; and, on the tergal aspect, between the scutum and 

 scutellum"; on the sternal aspect, between the sternum and 

 sternellum. 



The line of union between the episternum and epimeron is 

 the apodeme, which is an invagination of the body wall. The 

 division on the sternal aspect of most insects is not easily 

 recognized, but it shows plainly on the metathorax of Steno- 

 pelmatus and the nymph of Pteronarcys. As the apodemes 

 show the division of the pleural subsegments, so the furca 

 shows their division on the sternite. 



Embryologists have also proven that each segment "is com- 

 posed of a median and two lateral fields, and that the append- 

 ages are developed as evaginations of the lateral fields." 



III. Sclerites Belonging to Each Head Segment. 

 1. Ocular or Protocerebral. 



Tergal sclerites. No trace. 



Sternal sclerites. Front. Contains ocelli which are inner- 

 vated by protocerebrum. Decided ridge in Mantis front may 

 indicate sternum and sternellum. 



