104 INSECT PHYSIOLOGY 



place in the ovary and the embryo is already developing by the time 

 the egg is laid. In various Tachinid flies the eggs contain fully formed 

 larvae and these escape from the chorion during oviposition (ovo- 

 viviparity). In Sarcophagidae and some Oestidae, such as the sheep 

 bot, a smaller number of eggs are present and these may hatch inside 

 the 'uterus', as the enlarged lower part of the common oviduct is 

 called. And in other Diptera, such as Musca larvipara or Mesem- 

 brina meridiana, only one large egg passes at a time to the dilated 

 uterus ; it hatches when it is laid. In Hylemyia strigosa the larva has 

 already reached the second instar before it leaves the egg. 



In none of these cases does the embryo receive any nourishment 

 from the mother during its stay in the uterus. But in the tsetse-fly 

 Glossina and in the Pupipara the larva, which hatches in the uterus 

 from an egg of normal size, is nourished until fully grown by special 

 'milk' glands - modified accessory glands which branch throughout 

 the abdomen. These larvae are matured singly, breathing by the ex- 

 trusion of their posterior spiracles through the female opening; they 

 are not deposited until they are ready to pupate. In some insects, 

 such as the parthenogenetic forms of Aphids, embryonic develop- 

 ment continues in the ovary: the embryo, enclosed in very delicate 

 membranes, is nourished by the follicular epithelium until embryonic 

 development is complete. These are the most familiar of 'viviparous' 

 insects. 



'Polyembryony' is an unusual mode of asexual multiplication 

 which occurs in some parasitic Hymenoptera. After the maturation 

 of the egg the polar bodies may divide amitotically to form a nutritive 

 sheath; meanwhile the egg nucleus undergoes complete cleavage (for 

 these eggs contain almost no yolk) and the separated cells each de- 

 velop into embryos. In this way two or four embryos may arise from 

 a single egg in some species; anything from a hundred to two 

 thousand in others. 



In a few insects there is precocious reproduction in the larval 

 stages. The classic examples occur among Cecidomyidae. The pheno- 

 menon, which is known as 'paedogenesis', was first observed by 

 Wagner in 1861 in larvae of Miastor. The oocytes develop partheno- 

 genetically within the ovarioles until a number of daughter larvae are 

 produced. These are set free into the body-cavity where they rapidly 



