February 1, 1895.] 



KNOWLEDGE 



31 



FicJ. 1. — Hessian Fly (Cecidomi/ia des 

 inictor). (After Ormerod.) 



of its first arrival. The Hessian fly is so called because of 

 a tradition I that it was introduced into America by the 

 troops from Hesse, who were sent over to assist the British 

 forces in the year 1776, in the war of American indepen- 

 dence. The idea was that the insects were unconsciously 

 brought over, in their immature condition, amongst the 

 straw used by the troops. The Hessian troops conducted 

 operations in Long Island, and it was in this spot, 

 and immediately after this time, that the fly is said 

 to have appeared in great numbers. However this may 

 be, it did not attract general notice in Europe till long 

 after that date, for, according to Mr. Inchbald, its first 

 record for the Continent is its occurrence in Minorca 

 in 1834. 



The fly (Fig. 1) is gnat-hke in shape, with blackish head 

 and thorax, and pinkish abdomen, variegated with black 



spots. The wings, 

 a single pair, are 

 dark, except at the 

 base, where they are 

 pink, and clothed 

 throughout with 

 black hairs. Behind 

 them are a pair of 

 minute pink 

 knobbed stalks, 

 representing the 

 usual "balancers" 

 of a dipterous insect. 

 The antennse are 

 composed of seven- 

 teen joints, and like those of gnats in general, are beset 

 with circlets of hairs. The legs, which are comparatively 

 long and thin, are pale, with black hairs. The wing has 

 very few nervures, a short one near the front edge, then one 

 running longitudinally right across the wing, and, lastly, 

 one nearer tlae inner margm, forked at the tip. The female 

 insect is about one-third larger than the male, and also 

 darker in colour. It is a retu-ing insect, and as becomes 

 so destructive a creature, does not court observation, but 

 likes to retreat into the privacy and shelter of leaves or 

 holes in the ground, resting on the soil, where its dark 

 colour prevents it from being easily seen. The males are 

 more open in habits. The flies may be found twice in the 

 year, in spring and autumn ; the spring specimens being 

 hybernated ones of the year before, and the autumn brood 

 the progeny of these, which have passed through their 

 mitial stages during the summer months. 



As soon as the female fly has mated, she at once com- 

 mences the work of laying her eggs. According to Mr. F. 

 Enock, who has carefully worked out the hfe-history of this 

 pest, as recorded in the Transactions of the Entomolof/ical 

 SociHy, there will be from one hundred to one hundred and 

 fifty of these, and as they have to be distributed over parti- 

 cular leaves of suitably selected plants, the business of 

 getting them all properly placed is a somewhat exhausting 

 one, and not unfrequently ends in damage to the mother, 

 to the extent even of the loss of some of her lees. Most of 

 the eggs are laid on the youngest, i.e., the la'st developed 

 leaves of young plants, and on the inner side of the leaf, 

 in the grooves between the veins, a small number being 

 placed on each leaf, and these not all in one spot, but in 

 small clusters or sometimes singly. They are usually laid, 

 moreover, in such a position that the larva's head, as soon 

 as it hatches, will point towards the base of the leaf, the 

 object of the arrangement being apparently to give the 

 young larva the least amount of exertion in reaching its 

 final resting-place on the stem at the base of the leaf. ° Of 

 course the eggs are exceedingly minute. They are of a 



yellowish or reddish colour at first, but become darker as 

 the contained larva matures. 



The larva (Fig. 2, a) is a footless maggot, with fourteen 

 segments to its body, pale and transparent at first, but 

 afterwards becoming of a chestnut-brown colour, and 

 finally attaining a length of about the eighth of an inch. 

 When hatched, it has a toilsome journey before it ; it 

 must travel, by the workmgs of its muscles, over a distance 

 of some two or three inches down the leaf, from the spot 

 where its mother placed it, till it gets between the 

 ensheathing part n 

 of the leaf and 

 the stem it 

 clasps. Some 

 four hours wiU 

 be consumed in 

 this arduous 

 journey, and at 

 the end of this 

 time it turns 

 round so that 

 its mouth faces 

 the stem instead 

 of the leaf, as 

 has hitherto 

 been the case, 



Fig. 2. —a. Feeding lavTa of Hessian Flv 

 (magnified 8 diameters) ; B. Head of 

 ditto, with anclior process (magnified 36 

 diameters); c Pupa of Hessian Fly (mag- 

 nified 8 diameters) . (After Enock.) 



but it still remains with its head downwards. As soon 

 as it reaches this position, concealed between the leaf 

 and the stem, its journeys are over ; it has arrived at the 

 spot where it is to spend the rest of its life till it becomes 

 a perfect fly. It therefore at once sets to work feeding ; 

 attaching its mouth to the stem, it sucks out the juices of 

 the plant, and this it continues to do till its feeding life is 

 over. The consequences of the attack are, after a time, 

 recognizable Ln the bent and trailing stem, which falls 

 over just above the position of the larva (Fig. 3). 



Now comes the strangest part of the story. The creature, 

 it will be remembered, is a dipterous insect, and the 

 maggot-like larvee of a large proportion of this order do 

 not cast the last larval skin, but become a pupa irithin the 

 old skin, which hardens and darkens into a neat barrel- 

 shaped form. This barrel-like body obviously is not the 

 true equivalent of the chrysalis of an ordinary insect ; the 

 genuine chrysalis is to be sought for inside, and the old 

 larval skin serves the pui-pose of a cocoon, protecting the 

 enclosed soft-bodied insect from whatever agencies might 

 be disastrous to it. A pupa thus enclosed is said to be 

 coarctate, and the whole object, skin and all, is often called 

 a puparium, to distinguish it from a true pupa. Thus, 

 what seems to be simply a chrysalis is in reality that and 

 something more. Now, in the case of the Hessian fly, a 

 further compHcation is introduced. When the larva has 

 reached its full size, it ceases feeding, and its skin hardens 

 and darkens, and separates from the enclosed being as 

 usual ; thus far there is nothing remarkable. The enclosed 

 insect in this case, however, is not a pupa, but still a larva 

 — a fasting larva, it is true, but nevertheless a larva in 

 form, and destined to undergo further changes before the 

 pupa stage is reached. It should, therefore, be called, as 

 Prof. Riley has suggested, not a coarctate pupa, but rather 

 a coarctate larva ; the puparium is not the last skin of the 

 larva, but the last but one. The outer skin is now of a 

 dark chestnut-brown, and this stage is often spoken of as 

 the " flax-seed " stage, from the resemblance of the 

 coarctate creature in shape and colour to a flax-seed. In 

 this same condition, so far as external appearance is 

 concerned, the insect will remain for at least five or six 

 weeks, and if it belong to the autumn brood, the condition 

 will be prolonged throughout the winter. 



