426 THE INVERTEBRATA 



In the embryos of most generalized insects only, are coelomic sacs 

 present in all head segments. 



In addition to compound eyes there are simple eyes or ocelli, of 

 two kinds. Lateral ocelli are usually the sole type of eye in larval 

 insects and represent the larval counterparts of the compound eyes 

 which function in the adult. Dorsal ocelli on the vertex of the head 

 of adult insects are structures distinct from the lateral and co-exist 

 with compound eyes. The ocellus consists of a single cornea, a trans- 

 parent area of cuticle which usually forms a lens-like body, the cells 

 which secrete it, and the visual cells arranged in groups, the retinulae, 

 having in the centre the optic rod or rhabdome. The compound eyes 

 (as described more fully in the section on the Arthropoda) possess a 

 cornea which is divided into a number of facets; corresponding to 

 each facet is a group of visual cells, the ommatidium. The current 

 theory of mosaic vision states that each ommatidium, isolated from 

 its neighbours by a coat of pigment, conveys to the retinula at its base 

 only such rays of light as travel parallel to the axis of the ommatidium. 

 The total impression is that of a mosaic composed of as many separate 

 pictures as there are ommatidia, every picture different from its 

 neighbours, but all combining to form a single "coherent" picture. 

 The compound eye has probably the advantage that it can detect 

 movements of the smallest amplitude. It gives, however, only a vague 

 idea of the details of objects, for there is no focussing apparatus and 

 only objects very close to the eye can be perceived clearly. In some 

 insects the eye is divided into two parts : a dorsal with coarse facets 

 which probably only serves to detect variation in illumination, and a 

 ventral with finer facets which gives fairly definite images of objects. 

 Possibly in some insects the first function in night vision the second by 

 day. It must also be mentioned that experiments show that many 

 insects can distinguish colours. The development of flower colour and 

 pattern is generally supposed to have taken place simultaneously with 

 that of the aesthetic senses of insects. 



The antennae are a pair of appendages consisting usually of many 

 joints. They are sometimes filiform but may show complicated varia- 

 tions in structure. In all cases they carry sense hairs, particularly 

 those which serve an olfactory function ; it is well known that in some 

 insects the removal of the antennae or coating them with paraffin wax 

 destroys the olfactory sense, but this is not always the case. 



The mouth is bordered dorsally by the labrum, a median plate or 

 sclerite which is underlain by the membranous roof of the mouth — 

 the epipharynx. 



The first two pairs of mouth parts, mandibles and first maxillae, 

 lie at the sides of the mouth, while the second maxillae, invariably 

 fused together, bound the mouth posteriorly, and are known as the 



