262 ANIMAL LIFE 



coloured and hardened. Within this, its organs undergo 

 profound changes, and new ones appear. Two pairs of 

 horns project at the head-end and form the respiratory 

 openings of the pupa. The tail severs its connection 

 with the body. The wings, legs, and elaborate pro- 

 boscis appear, and in favourable seasons the fly is 

 ready to emerge in eight to ten days from the com- 

 mencement of pupation. 



If we now turn to the terrestrial larvae of the 

 two-winged flies we find that they affect such a great 

 variety of habitat and food, and undergo their develop- 

 ment at such varying rates, that a whole volume 

 would not exhaust the subject. 



There is every transition, from the terrestrial to 

 the aquatic habit, from the free-living to the purely 

 parasitic mode of life. The possibilities of nature 

 seem to have been almost exhausted by these insects 

 in their efforts to fill every available niche that shall 

 afford nourishment for the growing period, retirement 

 for the development of the pupa, and exercise for the 

 instincts of the imago. In particular the value of 

 heat has been utilised to the utmost as a means of 

 assisting growth and development, and so producing 

 swarm after swarm in a single summer. 



The many tribes of blood-suckers use this stimulating 

 nourishment to force their progeny. The parasitic 

 larvae hasten their development in the warm hides 

 of flocks. When these hot-houses fail, cold-blooded 

 animals, vegetable juices, fungi, pools in hollow trees, 

 serve as substitutes. Only one advance seems possible 



