DEVELOPMENT OF INSECTS 



395 



The store of spermatozoa received by the female, and kept within 

 the receptaculum seminis, often lasts for a long time — for two or three 

 years in some queen-bees. 



Parthenogenesis, or the development of ova which are unfertilised, 

 occurs normally, for a variable number of generations, in two Lepidop- 

 tera and one beetle, in some coccus insects and aphides, and in certain 

 saw-flies and gall-wasps. It occurs casually in the silk-moth and several 

 other Lepidoptera, seasonally in aphides, in larval life in some flies 

 {Miastor, Chironomus), and partially or " voluntarily " when the queen- 

 bee lays eggs which become drones. 



A few insects hatch their young within the body, or are " viviparous." 

 This is the case with parthenogenetic summer aphides, a few flies, the 

 little bee parasites Strepsiptera, a few beetles and cockroaches. 



Development of the ovum.— The tubes which com- 

 pose the ovaries and lead into the oviducts begin as thin 

 filaments, the ends of which are usually connected on each 

 side. These thin filaments consist of indifferent germinal 

 cells, all of them potential ova, and of mesodermic epithelial 

 cells, which form the ovarian tubes, etc., and are connected 

 anteriorly to the pericardial wall. 



But in most cases only a minority of these cells be- 

 come ova, the others become nutritive cells which are 

 absorbed by the ova, and follicle cells which line the 

 walls of the ovarian tubes and help to furnish the egg- 

 shells. 



There may be, indeed, ovarian tubes without nutritive 

 cells {e.g. in Orthoptera), and then each tube is simply a 

 bead-like row of ova, which become larger and larger 

 as they recede from the thin terminal filaments and ap- 

 proach the oviducts. In other cases the bead-like row 

 consists of ova alternating with clumps of nutritive cells 

 {e.g. in Hymenoptera and Lepidoptera). In other cases 

 the nutritive cells mostly remain in the terminal region, 

 but their products pass down to the receding ova. 



As there are numerous ovarian tubes in each ovary, 

 and as the same process of oogenesis is going on in each, 

 numerous eggs are ready for liberation at the same time, 

 and are simultaneously discharged into the oviduct of each 

 side. 



The eggs are relatively large and contain much yolk. In 

 a few cases yolk is almost absent, as, for example, in the 

 summer eggs of the Aphides, which are hatched within the 

 body, and in some forms where the young are endoparasitic. 



