INSECTA 383 



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

 present in all head segments. 



There are two kinds of eyes, ocelli or simple eyes and compound or 

 faceted eyes. The ocellus consists of a single cornea, a transparent 

 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. These ocelli are usually the 

 sole type of eye in the larval insect and also coexist with compound 

 eyes in the adult. 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 omma- 

 tidia, 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 mandibles represent the basal joint of the crustacean 

 limb, and correspond in structure and function to the mandibles of 

 the Crustacea but never possess a palp (Fig. 287). 



The i^^ and 2nd maxillae are other paired appendages which show 

 features of resemblance to the corresponding limbs of the higher 

 Crustacea. In the fusion of the 2nd maxillae to form a single plate, the 



