32 



KNOWLEDGE. 



[February 1, 1895. 



But meanwhile considerable changes are taking place 

 in the interior of the " flax-seed," and to understand these 

 we must just consider for a moment the position of the 

 larva, and the difficulties that lie in the way of its 

 emergence into the open air, when it shall have reached its 

 perfect condition. In the first place, it will be remembered 

 that the larva is jammed in between the stem and the 

 sheath at the base of the leaf ; further, it is placed with 

 its head downwards ; and yet again, its back is turned 

 outward, while its ventral surface faces the stem. Each 

 of these an-angements, admirable enough for the creature's 

 needs while it was a feeding larva, is the exact opposite of 

 what would be necessary to secure for the perfect fly an 

 easy exit. It would need to be nearer the open, with its 

 head upwards and facing outwards, if it were to be at all 

 suitably placed for easily making its way into the open 

 air ; for any advance from its present position would 

 only take it further from the light, even if it had the 

 ability, which it has not, of piercing the hard wall of 

 the stem. A complete revolution has, therefore, to take 

 place, and the way in which this is done is one of the 

 most marvellous features in the life-history of this curious 

 insect. 



If the "flax-seed" be opened, the larva within is seen to 

 have a curious little appendage near the mouth on the 

 under side, shaped like a rod forked at one end (Fig. 2, b), 

 and capable of being moved about at the plain end as upon 

 a pivot. This curious object has been called the " anchor 

 process," or "breast-bone," and its use 

 was for a long time problematical. To 

 Mr. F. Enock belongs the credit of dis- 

 covering the use to which it is put, and a 

 very strange use it is. We have seen 

 that the larva needs to completely reverse 

 its position if it is to stand any chance 

 of escaping from its imprisonment, and 

 it uses the "anchor process" in order to 

 do it. By pressing the forked end of 

 this, time after time, against the wall 

 of the so-called puparium, or larva- 

 case, and using it as a lever or prop, 

 the creatm-e is able, by suitable muscular 

 effort, to work itself gradually round inside 

 the narrow case, bending itself double at 

 the bottom of it, and finally turning a 

 complete somersault, so that its head is 

 at the top of the case and faces outwards. 

 Its position is thus greatly improved, 

 though, of course, it is stiU enclosed 

 within the " flax-seed," which has 

 remained as it was, while its inmate has 

 turned upside down, or rather, right side 

 up ; it is, moreover, still jammed in 

 between the leaf and the stem (Fig. 3) ; and further, it is 

 still a larva, and a good deal has yet to be done before it 

 becomes a Hessian fly. 



After a time, the larva, or more correctly the coarctate 

 larva, as it lies within the hard, dry skin of the 

 so-called puparium, sheds its skin and appears as a pupa, 

 still, of course, enclosed within the " flax-seed." This 

 pupa is shorter than the larva, and soon shows the outline 

 of legs, wings, head, eyes, &c., in its upper part (Fig. 2, c) ; 

 it has, moreover, a brown projection something like a 

 parrot's beak near the position previously occupied by the 

 '•anchor process." The pupa is at first white, but 

 gradually darkens in colour tDl it is mature. The insect 

 does not remain more than about twelve days in this 

 state. At the end of that time, it bursts through at the 

 upper end of the "flax-seed," and works its way up 



Fig. 3-Tertical 

 section through 

 bent barley stem, 

 showing '■ flax- 

 seed" in position 

 (magnified .5 dia- 

 meters). (After 

 Enoct.) 



between the leaf and stalk, leaving its old shell in the 

 position it has always occupied. It is still a pupa, and 

 another moult has yet to take place before its course is 

 ended. But this final change cannot be made in the 

 narrow quarters between the leaf-sheath and the stem, or 

 the delicate little fly would be imprisoned and unable to 

 escape. The chrysalis itself, therefore, must work its way 

 through the leaf, and it uses its beak to do this. By 

 means of the hard point it cuts a slit in the leaf, just 

 large enough to admit of its pushing through its fore 

 part, which therefore projects into the outer air. The 

 pupa skin now splits along the back near the head, and 

 the fly gradually works its way out of the opening, the 

 hinder part of the pupa-case being meanwhile tightly 

 nipped by the leaf, and held in position so as to give pur- 

 chase to the struggling fly. 



From the above description it will be seen that the fly, 

 as it struggles out of its case, has its face turned towards 

 the open air, and its back towards the stem, where alone 

 support can be found for a weak and struggling insect ; 

 and the question naturally arises, how does it manage to 

 complete its extrication without falling over ? for, through 

 the softness of its wings, it is not prepared to take flight 

 immediately on becoming free from the pupa-case. Mr. 

 Enock has shown how this difficulty is met. In a specimen 

 that he watched extricating itself, he points out how it 

 first released the head, thorax, antenns, and part of the 

 abdomen ; then followed one wing, then portions of the 

 legs, then the other wing, and then the first pair of legs 

 became quite free. Now it was projecting considerably 

 from the pupa-case, with only the last two pairs of legs 

 still left in their investing skin to hold it upright, and it 

 seemed to be on the point of toppling over ; but, continues 

 Mr. Enock, "just when the second pair of legs were quite 

 free, it swung them about until one caught hold of the 

 stem, towards which the fly immediately drew itself until 

 it had a firm hold, when it quickly withdrew the remaining 

 pair of legs, becoming quite free and walldng along on the 

 under-side of the bent straw ; here it hiing attached by 

 its first and second pairs of legs, with the abdomen per- 

 pendicular, and the claspers on the tail in the same 

 position ; the stick-like wings were flapped together over 

 its back a number of times, and in a quarter of an hour 

 were fully expanded, and then crossed in position ; the 

 anal claspers turned up over the back, the tips bent down- 

 wards, and at 8.30 p.m., or exactly an bom- and a quarter 

 after the straw was split, the fly made its first flight." 

 Needless to say, all these observations had to be made with 

 the assistance of a lens. 



The Hessian fly being such a formidable pest, it becomes 

 a matter of the utmost importance to determine in what 

 way its ravages may best be checked. In the case of this 

 creature, as in that of most other insect pests, there can be 

 little doubt that the most efficacious remedy is that which 

 Nature herself provides, in the form of parasites, which, 

 feeding upon the body of the larva, prevent its reaching 

 the perfect state, and therefore cut oii" all chance of its 

 being followed by a line of descendants. Several of these 

 parasites are known. They are all minute kinds of 

 hymenopterous insects, belonging chiefly to the family 

 called ChaUiduleB, near allies of the ichneumon flies. 

 They are chiefly of a shining blackish colour, with a 

 greenish tinge. 



As the result of two years' work, Mr. Enock obtained a 

 total of four thousand four hundred and fifty-one straws 

 infested with the Hessian fly, and bred from these seven 

 hvmdred and fifty-two flies and nine himdred and nine 

 parasites. As the number of parasites in general corresponds 

 with the number of larvae destroyed, it will be seen from 



