1910 ENTOMOLOGICAL SOCIETY. 35 



secrets one by one through tlie toil of men of science, and it is by the application 

 of the knowledge thus gained that we shall be able to avoid that great rod of 

 chastisement of Nature, disease, which is inflicted on those who disobey or trans- 

 gress her unwritten laws. 



And while we have these insects arrayed against us there are others allied 

 to them v^hich, by assisting in the maintenance of a balance in nature take up 

 their position on our side. These are the Tachinid flies, many of which, to the 

 casual observer,, have a superficial resemblance to the house-fly. There are a very 

 large number of species of Tachinidae and considerable variation in size and struc- 

 tural characters. The larvse of most of them are parasitic on lepidopterous larvae, 

 and in certain cases these parasites increase to so great an extent that they may 

 suppress completely an exceptional increase of caterpillars. It is largely owing 

 to these parasites and the Ichneumons that our vegetation is not completely eaten 

 up by lepidopterous larvae. The Tachinid maggot feeds inside the caterpillar 

 and gradually destroys the tissues of the host. In some cases the maggot leaves the 

 caterpillar before it pupates, in other cases the caterpillar pupates and the Tachinid 

 maggot pupates inside the cocoon or pupal cell. Their life-histories are extremely 

 diverse and this is frequently found to be the case in the life-histories of the 

 members of a group of animals which has assumed parasitic habits, whether it be 

 insects, Crustacea or more lowly organisms. Parasitism induces profound changes 

 in the life-history of animals, and we find great variations in so small, compara- 

 tively, a group as the Tachinidae. Whereas some of the insects deposit their eggs 

 upon the skin of the caterpillars, from which position on hatching the larvee bore 

 into the interior of the host; others deposit their eggs upon the leaves of the plant 

 as, I believe, is the case in Ugimyia sericaria, Eond, which is the cause of the 

 most serious " Ugi " disease of the Japanese silkworms. The caterpillars, on feed- 

 ing on the leaves, take the eggs in biting off the portions of leaf to which they 

 are attached, into their digestive tracts where the maggots emerge and bore through 

 into the tissues of the body. A third class is exemplified by the species Eupeleteria 

 magnicornis, Zett, which, as Townsend has shown, deposits living maggots not 

 on the caterpillars, but upon the green shoots, leafribs, etc., on which caterpillars 

 were present and usually on the silken thread spun by the caterpillar on its trail 

 over the plant. It will be recognized that the first method is the one in which 

 the maggot is least sure of success in entering the host, for it not infrequently 

 happens that the caterpillar moults before the eggs of the Tachinid hatch and 

 thus rids itself of the danger. So that although we may find Tachinids depositing 

 their eggs freely upon caterpillars, it is unsafe to predict beneficial results. Howard, 

 for example, mentions a case in which 226 moths and only four Tachinid flies were 

 obtained from 235 caterpillars of the Gipsy Moth, upon each of which 1 to 33 

 Tachinid eggs had been observed. In another case, 252 caterpillars, all bearing 

 Tachinid eggs, were reared and not a single fly emerged. These cases illustrate 

 well the advantage to an insect larvae of such a method of growth by the moulting 

 of the old skin. Once inside the caterpillar the maggot bores its way into the body 

 cavity or blood cavity and immediately attaches the breathing pores at the posterior 

 end of its body to one of the breathing pores of spiracles of its host and thus obtains 

 its air direct from the outside. In this position it remains during the whole of its 

 larval life, until it is full grown. It moults twice and the old skins remain attached 

 at the posterior end of the body as the maggot does not leave the spiracle of the 

 caterpillar to which it has once attached itself, until it flnally leaves it to pupate. 



Allied to these insects are the Anthomyidae, some of whose larvs are the de- 

 structive root-maggots, which feed on the roots of many vegetables, such as cabbages, 



