250 MANUAL OF ZOOLOGY sect. 



ganglia are closely approximated. There are sometimes 

 three pairs of thoracic and as many as eight of abdominal 

 ganglia in the adult insect, but in many there is a greater or 

 less degree of concentration of the ventral ganglionic chain. 



The most highly developed organs of special sense are the 

 large compound eyes which are situated on the sides of the 

 head. The surface of the compound eye is marked out, as 

 in the case of the crayfish, into a great number of minute 

 hexagonal facets, each of which corresponds to one of the 

 elements (ommatidia) of which the eye is made up. In 

 addition to the large compound eyes most insects have 

 simple unfaceted eyes or ocelli. In a few insects eyes are 

 entirely wanting. 



The antenna? and palpi are the organs of touch, and these 

 appendages seem also to be the seat of the olfactory sense. 

 The sense of taste is probably also developed in some 

 insects in minute papillae on the epipharynx which forms 

 the roof of the mouth and also at the base of the maxillae. 

 Special nerve-endings supposed to be auditory occur in 

 various parts of the body in some cases. 



The sexes are always separate, and the males and females 

 are very commonly distinguishable from one another by 

 various modifications of form and of coloration. Some 

 insects, such as the Aphides and bees and wasps, present us 

 with the unusual phenomenon of parthenogenesis, i.e., ova 

 are formed, as in ordinary female insects, in organs corre- 

 sponding to the ovary of the latter, and these are developed 

 without fertilisation. In the case of the Aphides, an autumn 

 generation of completely developed males and females is 

 followed by a spring generation consisting entirely of 

 females ; these are both parthenogenetic and viviparous. 

 In the bees the workers (imperfectly developed females) 

 occasionally produce ova which, without fertilisation, de- 



