248 MANUAL OF ZOOLOGY SECT. 



hexagonal facets, each of which corresponds to one of the 

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

 addition to the large compound eyes, most Insects have 

 simple unfacetted eyes or ocelli. In a few Insects eyes are 

 entirely wanting. 



The antennae and palpi are the organs of touch, and these 

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

 A sense of taste is probably also developed in some 

 Insects, and special nerve-endings which seem 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 i parthenogenesis (see foot- 

 note, p. 1 80) ; i.e., ova are formed, as in ordinary female 

 Insects, in organs corresponding to the ovaries 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 consist- 

 ing entirely of females : these latter are both parthenogenetic 

 and viviparous. In the Bees, the workers (imperfectly de- 

 veloped females) occasionally produce ova which, without 

 fertilisation, develop into males (drones). In one or two 

 groups, including the Scale Insects (Coccidae) and Gall Insects 

 (Cynipidae), males are never developed, so that reproduction 

 is exclusively parthenogenetic. Pcedogenesis accompanies 

 parthenogenesis in certain of the Diptera, i.e., the larva 

 produces ova from which embryos are developed without 

 impregnation. 



The eggs when laid are protected from injury by 

 various methods ; they may be firmly fixed to the sub- 

 stratum, buried in the earth, or laid in the interior of certain 



