126 Object and Function of the Egg-stalk. 



these which confirm my theory. A glance at the 

 illustration (PL III) shows that Andricus noduli has 

 a very short egg-stalk with a relatively long ovipositor. 

 We must remember that Andricus noduli lays its ^^% 

 in August, in the cambium layer of the oak bark : but 

 there can be no want of oxygen in a plant tissue in 

 which metabolism is constantly going on, and it is not 

 therefore necessary to inhale oxygen by means of the 

 egg-stalk. The summer generations of many species 

 lay their eggs under the same favourable circumstances 

 as Andricus noduli, therefore their egg-stalks are 

 short. But this holds good only of species which lay 

 their eggs in leaves ; and we must except those which 

 prick the winter buds. As winter buds are properly 

 speaking resting buds, the surrounding plant tissues 

 can afford no nourishment, consequently the egg-stalk 

 must be long enough to come in contact with the outer 

 air. Against this explanation of the use of the egg- 

 stalk the fact may be advanced that in other hymeno- 

 ptera this contrivance is wanting, but it would not be 

 difficult to prove that in none of these cases is such 

 an arrangement needed. Thus all the Ichneumonidae 

 give over their eggs to the selected host, and in its body 

 they obtain all the nourishment they require. Many 

 saw-flies also sink their eggs into various parts of the 

 plant, but they do so always at a time when active 

 metabolism is going on. In the case of oak gall-flies on 

 the contrary, the eggs of most of the winter generations 

 are laid at a time when the plants themselves give no 

 signs of life, and when metabolism is dormant. 

 Another apparent objection is that inquilines closely 



