YOUTH IN BIRDS AND LOWER ANIMALS 59 



chief functions of the body are feeding and growth, whilst in the 

 adult condition the chief function is reproduction. As we have 

 seen, this division of labour may be carried so far that the adult is 

 incapable of feeding. There are some extraordinary cases, how- 

 ever, where reproduction takes place in the larval state, with the 

 result that the adult state is dropped altogether. The gall-midges 

 'are very small two-winged flies, the larvae of which live on the 

 tissues of plants, sometimes doing great damage, the Hessian fly, 

 which attacks wheat, being a familiar example. The adult females 

 of most of these flies lay eggs on the plant, and these hatch out 

 iinto minute grubs, which, after a time of feeding and growing, pass 

 ''through metamorphosis and produce the adult winged insects. In 

 one or two cases, however, it has been found that the ovaries are 

 developed actually in the larvae and that these produce young which 

 live on the tissues of their parent and finally leave it by boring a 

 hole through the skin. The parent in such a case dies without 

 having become a perfect insect. 



Similar instances of reproduction before the larval state has been 

 passed through occur as rare exceptions in several groups of the 

 animal kingdom, but the best-known examples are found in the 

 batrachians. The youthful stage of most of these animals, as I 

 have described in Chapter II, is passed in water, the young animals 

 being tadpoles. Usually the tadpole changes into the adult condi- 

 tion long before the approach of winter. It sometimes happens, 

 however, that the metamorphosis is delayed, and the animals, 

 growing far beyond the usual size, live through winter in the 

 tadpole condition. Such a result has been produced in a number 

 of frogs and toads, including both the edible frog and the common 

 grass frog, the common toad and the South European tree-frog, one 

 of the methods adopted being to place a grating below the surface 

 of the water, so that the tadpoles cannot emerge and have no 

 access to air. These cases are only unusual prolongations of the 

 duration of youth, and the abnormal tadpoles eventually either die 

 or pass through their metamorphosis and become adult. In some 

 of the urodeles, those batrachians which retain the tail throughout 

 adult life, a further stage has been observed. Examples of the 

 common newt, the Alpine newt and the crested newt have become 

 adult, and have laid fertile eggs which duly developed, before they 

 had passed through metamorphosis ; and there is reason to believe 

 that such an abnormal state of affairs occurs as a regular event 

 in a number of instances. A celebrated case is that of the 



