2 S t e -t" *'!" j NATURAL HISTORY. 



the larvae of which are all parasitic. In this family, which includes the largest species of the group, 

 we find a great variety of characters, but the insects composing it have the antenna; thread-like 

 or bristle-like, generally long and many -jointed, and the wings with from one to three complete 

 submarginal cells. The body is long and thin, and the abdomen shows at most seven segments. 

 The ovipositor issues from the extremity of the abdomen of the female. 



This enormous family of insects is at the same tims one of the most difficult to study 

 systematically, and although we know that the number of species must be very great, it is almost 



impossible to estimate what it may be. It has been calculated that 

 there are not less than 4,000 to 5,000 known species of Ichneumons, 

 but the data are very untrustworthy. They occur in all parts of the 

 world, and their importance in the economy of nature is very great. 

 WING OF ICHNEUMON. The females deposit their eggs in or upon the bodies of other insects, 



especially the larvae of Lepidoptera and plant-eating Beetles. The 



larvae hatched from these eggs feed upon the substance of their host, avoiding the vital parts, 

 so that the unfortunate animal goes on assimilating food for the benefit of the parasites dwelling 

 within him until he completes his term of larval existence, and sometimes even attains the perfect 

 state ; but sooner or later the parasites either break out of the body of their host, or spin their 

 cocoons within it, with a result that in either case is equally fatal. No stage of the insect's life is 

 safe from these active enemies; they attack all, from the egg to the imago, but the larvae receive most 

 of their attention. A great number of the species are confined to particular families of insects in the 

 choice of their victims, while others infest only particular genera or even species, and the charge 

 of parasites introduced into the body of an individual host is always proportionate to the relative 

 sizes of host and parasite. Thus the eggs of insects are attacked only by the smallest species of 

 Ichneumons, and only a single egg is deposited in them ; the larger Ichneumons also frequently 

 place only one egg in the caterpillars or other larvae which they attack, and the Ichneumon larva then 

 spins its cocoon within the emptied pupa case of its victim. On the other hand, many small species 

 deposit their eggs in large caterpillars or other larvae, and then the number of eggs is proportioned to 

 the size of the host, and the Ichneumon larvae either fill up the empty cocoon with a mass of close- 

 packed cocoons, or break out of the infested larva as it is preparing to change, and spin their cocoons 

 separately around it. But perhaps the most remarkable circumstance connected with this 

 parasitism is that the parasites are themselves subject to be attacked by parasites belonging either to 

 this family or to one of the succeeding ones, the females of these having the instinct to recognise the 

 presence within the host of a parasitic larva, and possessing the art of passing their eggs through the 

 integuments of the former into the latter. "We have thus in the history of these insects a series of 

 checks and counterchecks of the most astonishing complexity. As the Ichneumon larva uses up all 

 the material it derives from its host in building up its own body, it naturally grows pretty rapidly, 

 and the host may perhaps be stimulated to increased assimilation by the presence of hungry parasites 

 in its interior. The respiration of the latter is provided for in a curious way ; the principal tracheal 

 stems open at the hinder extremity of the body, and this is brought into connection with one of the 

 stigmata of the host, thus opening a free communication with the external air. 



The development of the ovipositor is very different in the females of different genera and 

 species of Ichneumons, and this stands in direct connection with their habits. In the females of 

 some forms the ovipositor scarcely projects from the extremity of the abdomen, whilst others 

 have a long, bristle-like organ two or three times the length of the body, and between these two 

 extremes every gradation occurs. The short ovipositors are possessed by species which deposit 

 their eggs in or upon easily accessible larvae ; the long ones characterise those which seek concealed 

 larvae, such as the grubs of wood-eating Beetles. 



The species of Ophion, Paniscus, and some allied genera which have long antennae, only two 

 submarginal cells, a compressed abdomen, and a very short ovipositor, possess a very curious history. 

 They deposit small stalked eggs (see figure), much resembling little seeds, upon the surface of various 

 caterpillars, and these eggs adhere to the skin of the caterpillars by little hooks at the extremity of the 

 stalk. After a time the egg splits into two valves, from between which a minute grub issues, and 

 proceeds at once to push its head through the integuments of the caterpillar, so as to feed upon tha 



