640 



CLASS IV. INSECTA. 



shell and are then said to be viviparous e.g. some Muscidae, 

 a few Oestridae, and the Pupipara amongst the Diptera, the 

 Strepsiptera, the Aphidae and certain Ephemoptera. All others 

 lay eggs, that is, are oviparous. 



Occasionally the egg develops parthenogetically i.e., without ferti- 

 lization. This takes place in the Aphidae, where parthenogenesis 



is limited to a few generations, and in 

 the Cynipidae to one. This alterna- 

 tion of parthenogenetic with sexual 

 reproduction is termed heterogamy. 

 Certain kinds of young of the 

 Psychidae, Tineidae, Coccidae and of 

 numerous Hymenoptera (Apidae, 

 Vespidae, Cynipidae, Tenthredinidae) 

 and others also arise from eggs which 

 have never been fertilized. The drones 

 of the social colonies arise in this 

 way. Chermes presents an example 

 of heterogamy, two oviparous but 

 different generations following one 

 another in the course of the year. 

 A winged form lasts through the 

 summer and from its eggs a wing- 

 less form arises in the autumn and 

 lives through the winter and is re- 

 placed by the first form again in the 

 spring. In many species of Chermes 

 the male is unknown. In certain 

 species of Tenthredinidae, Coccidae 

 and Cynipidae no male has yet been 

 found, and as far as we know repro- 

 duction in these species is entirely 

 parthenogenetic. In some insects 

 the unfertilized ovum always gives rise to one sex, it may be 

 male or it may be female. In a few Diptera paedogenesis 

 occurs, young being produced by a pupa or even by a larva. 



The eggs laid by an insect vary in number from a few as in the 

 Collembola and Psocidae to a million as in the Bee. They are 

 usually deposited in water or on or near the food, which may 

 consist of plants, animals or dung, on which the issuing larva 



FIG. 400. Micropyles (Mk) of insect 

 eggs (after R. Leuckart). a upper 

 part of the egg-shell of Anthomyia ; 

 b egg of Drosophila cellaris ; c 

 stalked egg of Paniseus testaceus. 



