ENTOMOLOGY 



to the skull (Fig. 35). The central plate, or body, lies between the 

 brain and the subcesophageal ganglion and under the oesophagus, which 

 passes between the anterior pair of arms. The tentorium braces the 

 skull, affords muscular attachments and holds the cephalic ganglia and 

 the oesophagus in place. It is not a true internal skeleton, but arises 



FIG. 35. Skull of a grasshopper, Dissos FIG. 36. Head of a gyrinid beetle, Dineutus, 

 teira Carolina, o, occipital foramen; t, t, to show divided eye. 



anterior arms of tentorium. 



from the same ectodermal layer which produces the external cuticula; 

 though authors are not agreed as to the details of the development. 



Eyes. The eyes are of two kinds simple and compound. The 

 latter, or eyes proper, conspicuous on each side of the head, are of com- 



FIG. 37. Agglomerate eyes of a male coccid, 

 Leachia fuscipennis. After SIGNORET. 



FIG. 38. Facets of a compound eye of 

 Melanoplus. Highly magnified. 



mon occurrence except in the larvae of most holometabolous insects, in 

 some generalized forms (as Collembola) and in parasitic insects. The 

 compound eyes (Fig. 41) are convex and often hemispherical, though 

 their outline varies greatly; thus it may be oval (Orthoptera) or triangu- 

 lar (Notonecta), while in the aquatic beetles of the family Gyrinidae 

 (Fig. 36) each eye has a dorsal and a ventral lobe, enabling the insect 

 to see upward and downward at the same time; so also in Oberea and 



