ICHNEUMONS AND THEIR ALLIES 103 



consume the leaves, or drill holes in the seed. Hence there 

 is no large class of animals which is so much afifected by the 

 annual cycle of vegetative changes. While many other animals 

 feed throughout the year, it is only such insects as dwell in 

 earth, water, or the trunks of trees that are active in winter. 

 The rest are usually obliged to complete the. feeding stage, as 

 well as the mating and egg-laying stage, within the summer. 

 Then they may feed with hardly a break, and multiply 

 viviparously, while still immature, as if to take the fullest 

 advantage of a time of plenty. The visits of insects are not 

 pure loss to the plants. Some flowers have come to depend 

 upon winged insects for the setting of their seeds. But the 

 balance of gain and loss is probably unfavourable to the 

 plants. They give up nmch valuable food-substance, and 

 undergo mutilations which are often serious and sometimes 

 fatal. The ravages of plant-eating insects are multipled when 

 man, for his own purposes, raises vegetables and trees by the 

 acre. Indeed, cultivation would often become impossible, if 

 nature had not provided remedies more effective than any that 

 we can contrive for ourselves. As the plant-eating insects in- 

 crease in number, insect-eating animals increase too. Natural 

 contrivance is impartially put forth for the advantage of the 

 plant, the herbivorous animal, and the carnivorous animal 

 alike. The plant multiplies its defences, the plant-eating 

 insect its weapons of attack, while the instincts of the pre- 

 datory bird or insect are sharpened. But the most effectual 

 check upon plant-eating insects is not the open warfare of 

 predatory species ; it is the insidious attacks of small and 

 artful creatures, themselves insects, which, betraying their 

 natural allies, rear their young at the cost of others. The egg 

 is laid in, upon, or near a feeding larva, and the grub which 

 issues from the egg is adapted by every refinement of organisa- 

 tion and instinct to devour the flesh and blood of a living 

 victim. Even the minute eggs of insects give opportunity to 

 still smaller parasites. There is a very numerous tribe of egg- 

 parasites, which lay their own eggs in the eggs of larger species. 

 The parasitic larva devours the yolk, and at length emerges as 

 a fly totally different from the one which provided the yolk. 

 So far is living at another's cost carried that there are among 

 insects many examples of parasites upon parasites. One such 

 case will be mentioned in this lesson. Parasitism upon living 



