Fig. 482.—Female Dyticus, laying eggs: A, ovipositor extended. B, egg of Notonecta, attached to stem of rush. C, egg of Dyticus, laid in excavation in rush.—After Régimbart, from Miall.
Insects as a rule arise from eggs which are laid in a great variety of situations, those species which are viviparous being exceedingly few in number compared with the class as a whole. It is noteworthy that Leydig has found in the same Aphis, and even in the same ovary, an egg-tube producing eggs, while a neighboring tube was producing viviparous individuals.[[77]] The viviparous species are confined to certain May-flies, the Aphidæ, Diptera (Sarcophaga, Tachinidæ, Œstridæ, and Pupipara), and to certain Coleoptera (Stylopidæ and some Staphylinidæ).
The number of eggs laid varies from a very few, as in the Collembola and in the Psocidæ, or 15 or even less in certain fossorial wasps, and from 20 to 35 in some locusts to many thousands in the social insects, the honey-bee laying by estimate over 1,000,000 eggs in the course of her life. Dr. Sharp thinks that from 50 to 100 may perhaps be taken as an average number for one female to produce. The eggs of insects with a complete metamorphosis are said by Brauer to be smaller in proportion to the parent than those laid by ametabolous or heterometabolous insects. In this respect the insects are paralleled by the birds, the highest forms laying smaller eggs than the water birds, ostrich, Apteryx, etc.
Fig. 483.—Eggs (e) of Hydrobius (?) and their capsules, from which the larva, Fig. 452, hatched.—Emerton del.
The egg, or ovum, when laid is not always ripe or perfect, but, as in those of ants, continues to grow after oviposition. Others are laid some time after the embryo has begun to form; and in the flesh-flies the larva hatches before the egg is deposited.
Fig. 484.—Egg-masses of Chironomus: A, string of eggs of C. dorsalis, divided into sections to show both sides. B, twisted fibres which traverse the string of eggs. C, egg-mass of Chironomus (sp). D, egg-mass of a third species. E, part of D, more highly magnified. F, developing eggs, two stages.—After Miall.
Insects as a rule instinctively lay their eggs near or upon objects destined to be the food of the larva; those of caterpillars on leaves, those of many flies on meat or carrion, those of Copris and other dung-beetles in dung, those of aquatic insects in water, while many oviposit in the earth or in plants (Fig. 482), or in the bodies of animals destined to be the hosts of the parasitic larvæ. As the eggs are preyed upon by mites and other animals, the contrivances and modifications of the mode of egg-laying, and the situations in which they are placed, are almost endless. Many insects lay their eggs in a mass, covered with a gummy substance; or those laid in the water, as the eggs of dragon-flies, caddis-flies, Chironomus (Fig. 484), etc., are enveloped by a jelly-like mass.