10 Transactions of the 



whicli worked their way out, one here and another there, 

 from different points in the caterpillar's soft body. However, 

 if we may judge from appearances, the latter was scarcely 

 conscious of discomfort, far less of pain, from the inroads of 

 its strange guests. Its manner of life underwent no change, 

 and it wandered lazily and aimlessly about its prison, as 

 heretofore. True, it ate no food ; but caterpillars, be it 

 remembered, take no sustenance for some little time before 

 either of their transformations : and not the least wonderful 

 part of parasite history is, that the date of the deposition of 

 the egg is so admirably timed that the grub is able to emerge 

 from the body of the caterpillar just when the fatty substance 

 on which it feeds would naturally begin to fail. In other 

 words, just when the animal would, if uninjured, begin to 

 assume its chrysalid dress. This exact ' iitness of things ' 

 runs through all the insect world. As far as observation has 

 extended hitherto, it is invariably found that there is no 

 waste either of time or matter in Nature's works. We have 

 an analogous instance in the case of sand wasps, which lay 

 up a store of spiders, flies, &c. for the use of their young. 

 Their manner of proceeding is this : — Each kind of sand 

 wasp has a penchant for a particular insect, and confines 

 itself to that. Thus, Philanthtis triangulum seizes on honey 

 bees, Mellinus arvensis on flies, &c. The economy of their 

 doings is illustrated by the fact that, on seizing a victim, the 

 sand wasp does not slay it at once, but only stings it in a 

 modified form, so as to paralyse it, and render it helpless. 

 And why? Because at this period there is an egg only in 

 the cell of the robber wasp. Were the victim, therefore, put 

 into the cell dead, it would dry up or become corrupt before 

 the young wasp could take advantage of the prepared 

 banquet. As it is, the unfortunate spider or fly remains 

 alive, though utterly unable to escape, until the little savage 

 for whose benefit it has been deposited emerges from its egg. 

 A second illustration of the accurate balance preserved by 

 Nature in the supply and demand for food among her 

 children is, that exactly when the victim is devoured, the 

 time has arrived for the larva of the sand wasp to pass into 

 its chrysalis state, in which it rests, as in a temporary grave, 

 with neither the desire nor the power to eat. 



To return to our ichneumon fly. It is a noteworthy 

 circumstance (aj^propriate, too, to the subject of supply and 

 demand) that, although it spends weeks, perhaps months, 

 inside the caterpillar, gnawing and devouring in every direc- 

 tion, it all the time carefully avoids iniuring any vital part, 



