REPRODUCTION 101 



receptaculum and enter the micropyle of the egg just as it passes the 

 receptacular duct. Very little is known about the mechanism of this 

 process. The egg is usually so orientated that the micropyle is direc- 

 ted towards the duct; and in some insects, notably the honey-bee, 

 there are muscular valves which can limit the number of spermatozoa 

 that escape from the receptaculum. But the factors which control the 

 entry of spermatozoa through the micropyle are very incompletely 

 understood. 



At the time of ovulation the nucleus of the egg has not yet under- 

 gone maturation; but the chromatin has already collected into chro- 

 mosomes and the first maturation spindle has formed. Maturation 

 seems to be stimulated by the entry of the spermatozoa. Several 

 spermatozoa usually enter the insect egg (polyspermy), but only one 

 of these is normally concerned in fertilization. As it approaches the 

 egg nucleus and, losing its tail, becomes converted into the male 

 pronucleus, the two maturation divisions of the female follow rapidly 

 upon one another. One of these is the reduction division which halves 

 the chromosome number. The two divisions result in the formation 

 of the polar bodies and the female pronucleus which then proceeds 

 to fuse with the male pronucleus to form the zygote. 



Factors controlling fertility and fecundity 



Insects with a short imaginal life copulate once only and lay their 

 eggs in a single batch ; others lay batches of eggs with intervals be- 

 tween and may copulate repeatedly; others, again, lay single eggs at 

 fairly regular intervals. In all these groups the egg production or 

 fecundity of the female and the fertility of the male are influenced by 

 a great variety of factors. 



Like other processes of metabolism, the rate of egg production is 

 markedly influenced by temperature, and usually egg production 

 ceases at low temperatures at which other physiological activities 

 continue normally. At the upper temperature range the male seems 

 to be particularly sensitive: males of Drosophila and Ephestia be- 

 come sterile if reared at temperatures only a few degrees above 

 their optimum. 



Nutrition, of course, has a powerful effect on reproduction, par- 

 ticularly on fecundity in the female. Some insects require water, 



