GROWTH AND TRANSFORMATION 195 



to display just as much activity as may correspond to its 

 manner of life or to the necessity of preparation for the final 

 moult. The details of structure and habit are largely 

 adaptive, and the course of the life-history of insects as a 

 whole suggests a great degree of plasticity in correspondence 

 with biological relations. 



The same conclusion as to a plasticity in the details of 

 development and correspondence to environmental needs 

 is suggested by many facts which confront the student as 

 startling exceptions to the normal progress of insect trans- 

 formation ; some examples of such may fitly close this 

 chapter. 



Reference has already been made to the summer genera- 

 tions of greenfly (Aphids) in which the eggs develop within 

 the mother's body so that the young are not hatched but 

 born. Many two- winged flies (Diptera) give birth to 

 active maggots instead of laying eggs. The Sheep-fly 

 {Oestrus ovis), for example, usually deposits tiny larvae in 

 the nostrils of the sheep, though sometimes according to 

 the weather conditions, as W. E. CoUinge (1906) has shown, 

 she lays eggs, whence later the maggots are hatched. The 

 big Flesh-fly {Sarcophaga carnaria) is constantly ** larvi- 

 parous," and so are many of the Tachinid flies whose 

 maggots feed as parasites in the bodies of other insects. 

 The female of a species of Compsilura is provided with a 

 sharp " larvipositor " by means of which she pierces the 

 body-wall of a caterpillar and thus places her offspring 

 safely inside, where they invade the wall of the stomach 

 and begin to feed. More rarely does the larva undergo 

 most of its growth within its mother's body ; the dreaded 

 African Tsetse-flies (Glossina) as well as certain Dipterous 

 insects (Hippoboscidae and Melophagidae) which suck blood 

 from mammals and birds, bring forth mature larvae that 

 pupate immediately after birth ; hence these last named 

 insects are often called '* pupiparous." Some of these in 

 correlation with their parasitic life are wingless, and there 

 are two most remarkable types of wingless Diptera, living 

 as '' guests " in the nests of termites in the African and 



