THE STRUCTURE OF THE HEAD. 19 



and several low Neuropterous forms, as the larva of Ephemera, 

 but chiefly the embryo of Diplax^ a dragon-fly, we have con- 

 cluded that there are seven such elemental segments in the 

 head of insects. 



That there are four corresponding to the jointed appendages, 

 i. e. the labium, or second maxillae, the first maxillae, the man- 

 dibles, and the antennae, seems indisputable. But where else 

 are we to look for jointed appendages in an insect's head ? We 

 must go out of the class of Insects and study the stalk-eyed 

 Crustacea, such as the Lobster, where the eye is supported on a 

 two-jointed stalk, which has been homologized with the limbs. 

 While, therefore, the eyes of insects are never "stalked," as in 

 the Lobster and Shrimp, they are evidently developed, as in 

 the Crustacean, upon a separate segment (or its rudiments), 

 which may be called the " ophthalmic ring," and which is, there- 

 fore, the fifth cephalic ring. In advance of the eyes are nor- 

 mally placed the three ocelli, though in the highest Insects (the 

 Diptera, Lepidoptera, and Hymenoptera) they appear to be 

 situated in the rear of the eyes. 



Each of these three ocelli is situated upon a distinct piece ; 

 but we must consider the anterior single ocellus as in reality 

 formed of two, since in the immature pupa of Bombus the 

 anterior ocellus is differently shaped from the two posterior 

 ones, being transversely ovate, resulting, as I think, from the 

 fusion of two originally distinct ocelli, and not round like the 

 other two. There are, therefore, two pairs of ocelli, and hence 

 they grow from the rudiments of a sixth and seventh ring 

 respectively. 



Now, since the arthropleural is the limb-bearing region in 

 the thorax, it must follow that this region is largely developed 

 in the head, to the bulk of which the sensory and digestiA'e 

 organs bear so large a proportion ; and as all the parts of the 

 head are subordinated in their development to that of the ap- 

 pendages of which they form the support, it must follow logi- 

 cally that the larger portion of the body of the head is pleural, 

 and that the tergal, and especially the sternal, parts are either 

 very slightly developed, or wholly obsolete. Thus each region 

 of the body is characterized by the relative development of 

 the three parts of the arthromere. In the abdomen the upper 



