THE STRUCTURE OF THE HEAD. 19 



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

 but chiefly the embryos of Diplax, Chrysopa, Attelabus, Nema- 

 tu&, and Pulex, we have concluded that there are four such ele- 

 mental segments in the head of hexapodous insects. 



On reference to fig. 57 it will be seen that there is a sternal 

 portion on the under side of the two posterior segments of the 

 head, and in the embryo of Attelabus we have seen sterna also 

 developed in the antennal and mandibular segments, so that we 

 may conclude that there are four segments in the head of all 

 six footed insects, corresponding to the jointed appendages, 

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

 dibles, and the antennae. Though having, in accordance with 

 the generally received opinions of Milne-Edwards, Dana, and 

 others, believed that the eyes of Crustacea, and therefore of 

 Insects, were the homologues of the limbs, and developed on 

 separate segments placed in front of the antennal segment, as 

 stated in the previous editions of this work ; I have, however, 

 on farther study of the subject, been led to reconsider the mat- 

 tir, and decide that the eyes are but modified dermal sense 

 cells, and in certain articulates developed on limb-bearing seg- 

 ments. Thus in the King Crab (Limulus) a pair of ocelli are 

 situated on the first segment of the body, and the large com- 

 pound eyes grow out on the back of the third segment, both 

 bearing limbs. In the embryos of all the insects yet exam- 

 ined, the eyes are groups of specialized cells of the skin which 

 grow out on the upper, or tergal, side of the same segment 

 which bears the antennae. In certain mites, as Hydrachna, and 

 its allies, the simple eyes are situated over the second pair of 

 legs, and at a considerable distance behind the head. Among 

 the worms, also, organs of sight, as in Polyoplithalmus, are 

 developed on each segment of the body ; or, as in certain Pla- 

 narians, scattered irregularly over the body. 



The three ocelli, when present, are developed after the eyes 

 appear. 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 transversely ovate, resulting from the 

 fusion of two originally distinct ocelli. There are, therefore, 

 apparently two pairs of ocelli. The clypeus and labrum are 



