38 



is not unlike a leech, both in appearance and movements. Having no eyes, it appears to be 

 placed at a great disadvantage in the search for food, but fixing the hinder extremity of its 

 body to the surface on which it is placed, it reaches as far as it is able to stretch first on one 

 side, then on the other. If no food is reached, it moves a little further, and then repeats the 

 same motions, and so on until it reaches a plant-louse, when at once it seizes its prey, holding 

 it up in the air, as shoven in fig. 20, until having sucked it empty, nothing remains ot the 

 louse but a shrivelled skin, and all this occupies scarcely more than a moments 

 time ; indeed it has been asserted that a medium sized larva will thus consume 

 a hundred jjlant lice in an hour. These larvae are semi-transparent, so much so 

 that the movements of some of the internal organs are plainly discernible through 

 the skin in one of the larger specimens. Their usual colour is whitish or green- 

 Fig. 20. ish, with white, yellow or dull reddish markings, or with a combination of these 

 down the centre of the back. When they have completed their growth, they fix themselves 

 to the surface of a leaf or a piece of bark and contract themselves to an oval form, which 

 gradually becomes hard and horny, and of a blackish colour, and within this shell the change 

 to a chrysalis takes place, and in due time, from it the perfect fly escapes. 



But the aphis has foes which attack it from within, as well as from without. Almost all 

 insects are infested more or less by parasites, and the aphides are no exception to this general 

 rule. There are several genera of tiny parasites which thus befriend man, all of which are 

 included in one group named A'phidiides. Dr. Fitch's remarks on this interesting group are 

 so mueh to the purpose that we quote them entire. He says : " These are all exceedingly 

 tmall insects, little exceeding the twentieth of an inch in length, and mostly with black bodies, 

 variously adorned with bright tawny yellow, and pale sulphur yellow bands and otlier marks. 

 One of these small Ichneumon flies, resembling a winged ant in appearance, may occasionally 

 be discovered busily at work among a colony of aphides. With her long thread-like antennas 

 stretched out in front of her, and rapidly vibrating, she approaches an aphis and touches it 

 gently, much like an ant when nursing these creatures. By this slight touch, she at once 

 ascertains whether the aphis has been previously visited. If it has not, she curves the tip of 

 her abdomen forwards under her, puncturing the body of the aphis and inserting an egg 

 therein. She then passes to another and another. From this egg hatches a minute worm, 

 which resides within the aphis, subsisting upon the juices which the latter extracts from the 

 plant. Thus it grows with the growth of the aphis, which furnishes the exact amount of sus- 

 tenance which the worm requires for bringing it to maturity. It is singular that the parent 

 Ichneumon fly knows if two eggs were deposited in the aphis the worms from them would 

 die for want of a due supply of food, and that by a mere touch with her horns, she is able to 

 ascertain which individuals have already been impregnated. Some of the species of Aphidius 

 are larger than others, and their ofispring consequently recj^uire a larger quantity of food ; but 

 each parent has the instinct to select an aphis of such size as will yield the amount of suste- 

 nance which its young requires. 



" By the time the worm has attained its growth, the aphis becomes so exhausted that it 

 dies. If it should now drop from the leaf to the ground, it would be liable to be found and 

 devoured by centipedes and other insects which feed upon the carcases of animals of this 

 class, and thus the worm within it would be destroyed. Nature has, therefore, so constituted 

 the aphis that in these circumstances it dies without a struggle or a spasm, with its beak in- 

 serted, and its claws clinging to the surface of the leaf, standing with its antenna3 turned 

 backwards, and its whole aspect so life like that in the infancy of my studies, I supposed these 

 were one of the varieties natural to the species with which they occurred. Their bodies are 

 remarkably plump and smooth, commonly clay-coloured, or the hue of brown paper, and the 

 aphis lions and other insects, which destroy the aphides, appear to pass by those which have 

 these parasites within them. Hence, where a leaf or twig has recently been cleared of plant 

 lice by their enemies, several of these ichneumonizcd individuals may frequently be found re- 

 maining upon it, dead and unmolested. In other instances, the whole colony of aphides ap- 

 pears to be exterminated by these parasites alone, the dead swollen bodies of tlieir victims 

 covering tlie surface of the leaves or twigs as closely as they can stand. The worm remains 

 within the body of the dead aphis during its pupa state. It then cuts a circular hole through 

 the dry hard skin, and comes out in its winged and perfect form." 



" These parasitic insects, which feed internally upon the aphides, are as efficient in de- 

 stroying them as the aphis-lions or any other class of their enemies. And it is truly wonder- 



