The sexes of insects are separate and may in some cases be distinguished externally, as in the bee, butterfly and cockroach. Conjugation is preceded by a sexual act during which spermatozoa are transferred from the male to the spermatheca of the female. With winged forms this generally occurs during flight. The sperms make their way to the genital pouch where conjugation occurs as the eggs are being laid. A secretion from the colleterial glands then covers each egg and hardens into an egg capsule on exposure to air. The young in the great majority of cases are larva? entirely unlike the adult form.
Certain insects, of which the hive bees and aphides are common examples, lay unfertilized eggs which can develop into adult organisms. This phenomenon is termed parthenogenesis. The queen bee before her mating flight sometimes lays eggs which always hatch out into drones or male bees. Since drones, workers and queens develop from eggs laid after copulation, it is clear that there must be some factor or mechanism at work. There is some evidence to show that when the queen bee puts her abdomen into a drone cell, since it is deeper than a worker cell, the duct of the spermatheca becomes closed and so the egg which is laid cannot be fertilized and must therefore develop into a drone. The differences between the queen and the worker are due to differences induring the larval stage—a grub fed for five days on royal jelly develops into a queen, whereas if fed for three days on royal jelly, followed by two days on bee bread, a worker results. In rare cases workers have been known to lay eggs, but they always develop into drones, for it is not possible for the worker to lay fertilized eggs, since mating between worker and drone does not occur. The worker is therefore a sexually imperfect female bee.
The following summarizes the essential points :—
Larva from fertilized eggs, I.e. zygotes, normally fed become workers. Larvae from fertilized eggs specially fed become queens. Larva? from unfertilized fertile eggs, I.e. female gametes, normally fed become drones.
In the case of broad bean aphis, the type of individual produced depends upon the type of female ; the type of food upon which she feeds ; the time of the year.
In autumn wingless females lay eggs on the spindle tree. These hatch the following spring into wingless females whichon the spindle and produce wingless young parthenogenetically. No eggs are laid, the young being produced alive. Several generations are produced in this way until the food supply begins to give out, when winged migrant forms are born. These fly to broad bean and other plants and, voraciously, produce enormous numbers of wingless viviporous females whilst the food lasts.
In the autumn winged males and winged viviparous females appear which migrate to the spindle tree. The latter produce parthenogenetically wingless females which copulate with the males and lay eggs which survive the winter that kills off the adult forms.
So rapidly do aphides multiply parthenogenctically that they are a serious menace to our crops, for certain species attack apple and pear trees, peas, beans, plum and rose. Fortunately heavy rain washes them away, cold checks reproduction and many insectsupon them.