August 1, 1895.] 



KNOWLEDGE 



177 



tails and the familiar cheese-jumper is of a somewhat 

 exceptional character ; the former have a forked spring 

 bent under their bodies, and the latter performs its spas- 

 modic jerks by coihng itself into a circle and then violently 

 straightening itself out, with the result that it is shot 

 about indiscriminately in a manner that defies calculation. 

 But in most of the other instances the mechanism is of 

 one uniform type, and consists of a highly developed mass 

 of muscles enclosed within the greatly enlarged thighs of 

 the hind pair of legs, and this may be considered as the 

 most usual type of structure associated with leaping habits, 

 finding its counterpart even amongst vertebrates in the 

 enlarged hind legs of the kangaroos and jerboas. 



This type of structure is exemplified, not only in the 

 familiar groups already enumerated, but also in another 

 set of leapers, less well known, and found amongst the 

 Coleoptera, or beetles. These leaping beetles form the 

 group known as Halticte, and it is this group of which we 

 are to speak in this paper. They 

 are almost all of very minute size, 

 and consequently not generally well 

 known ; nevertheless, many of them 

 are extremely abundant, and from 

 their ravages, some of them have 

 become of considerable commercial 

 importance. The best known and 

 the worst offenders are called by 

 farmers "turnip tleas," or ''turnip 

 flies," and as an example of the 

 havoc it is possible for them to work, 

 Curtis, in his " Farm Insects," 

 mentions a case reported from 

 Devonshire in 1786, in which it was 

 stated that damage to the extent of 

 £100,000 was done to the turnip 

 crop during an imusual visitation, in 

 that county alone. This group of leapers forms a homo- 

 geneous collection of vegetable feeders, amounting in 

 the British Islands alone to some hundred and twenty 

 species, which is not more than one-third of the number 

 that inhabit Europe at large. But as the majority of them 

 are attached to wild plants which are of no commercial 

 importance, the true pests form but a small percentage of 

 the total number of species. 



The strong family likeness that exists amongst all these 

 beetles makes it easy to obtain a good idea of the general 

 structure and habits of the group from the detailed examina- 

 tion of a single species. We will select as our typical 

 example one of the commonest of the group, the " turnip 

 flea," properly so called. It is a minute, hard- skinned 

 beetle, ranging from about one-twelfth to one-tenth of an 

 inch in length, with black body, bronze-black thorax, and 

 black wing-covers, each of which is adorned with a longi- 

 tudinal yellow stripe, and covers a large folded membranous 

 wing. This description, however, is not nearly minute 

 enough to correctly distinguish the species, for it would 

 apply equally well to several species which are very closely 

 allied, and difler from one another chiefly in such minute 

 points as the width and exact shape of the yellow stripe, 

 and the colour of the legs. The true turnip flea is called 

 Phiilhitreta nemnruiii (Fig. 1), but in all probability its 

 depredations are considerably exceeded by those of a far 

 commoner and slightly smaller species, called P. widulnta, 

 which is one of the most abundant insects in the country. 

 These two little pests and their allies belonging to the same 

 genus are specially attached to cruciferous plants, notably 

 the turnip, radish and horse-radish amongst cultivated 

 kinds. To anyone who has grown a bed of radishes it must 

 have been a familiar experience to note the leaves riddled 



Fia. 1. — Turnip Flea 

 ( PhifUotreta nemorum ) ; 

 niiignitlecl twclre dia- 

 meters. 



with multitudes of minute holes (Fig. 2), as though they 



had received a charge of small shot. This damage is the 



work of turnip fleas. If a bed of radishes so marked be 



carefully looked over, especially 



if the holes appear freshly formed, 



the eye will soon learn to detect 



a number of minute insects here 



and there, sitting on the leaves ; 



as soon as the hand approaches 



them with intent to capture, they 



suddenly pass into invisibility by 



springing up so quickly that the 



eye cannot follow the direction 



they take, just as would be the 



case with a true flea. 



The oval green eggs are laid 

 on the under side of the leaves, 

 but as the perfect insect itself is, 

 at the outside, no more than one- 

 tenth of an inch in length, it is 

 easy to see that the eggs must be 

 so minute that a search for them 

 over the wide area of a turnip 

 hopeless matter 



Fro. 2. —Portion of Radish 

 Leaf perforated bv Turnip 

 Flea. 



Fig. 3.-Clirysali 

 of Turnip Flea. 



field would be rather a 

 The best way to observe them is to place 

 some of the beetles in a glass tube surrounding a growing 

 plant in a flower-pot ; they will soon lay some eggs, and a 

 lens brought to bear upon the few leaves enclosed will be 

 able to discover them. The eggs hatch in about ten days, 

 yielding a little six, or rather, seven-legged caterpillar-like 

 being, which at once eats its way into the body of the 

 leaf; ensconced there, it proceeds to devour the soft cells 

 of the leaf tissue that lie between the two cuticles, thus 

 excavating a minute tunnel, along which it travels by 

 means of its six hook-like front legs and its terminal prop. 

 It takes rather less than a week over 

 this operation, and then leaving its 

 burrow, descends to the ground and 

 buries itself in the soil to the depth of one 

 or two inches, taking care to select a spot 

 near to the turnip root, so that the over- 

 hanging leaves may provide suitable shade 

 and preserve a proper amount of moisture 

 in the soil. While thus buried it becomes 

 a chrysalis (Fig. 3), which, small as it is, 

 exhibits all the details of the future beetle, 

 head, thorax, wing-covers, legs, antennse, neatly folded 

 beneath the investing pellicle. 



About a fortnight is sufficient for the necessary internal 

 changes to be completed, and then the new beetle issues 

 from its close-fitting garment and pushes its way to the 

 surface of the soil, ready to resume the attack on the 

 leaves which has for the past fortnight been intermitted, 

 but attacking them from without instead of, as before, 

 from within. Thus a period of rather less than five weeks 

 suffices for the beetle to pass through all its changes, from 

 the egg to the adult, though this must not be taken as a 

 measure of its lifetime, for it often Hves for several months 

 after becoming a perfect beetle. So short a period being 

 all that is necessary to carry the insect through its trans- 

 formations, and put it into the position of becoming the 

 founder of a family, it is not surprising to learn that 

 several broods are to be met with during the course of one 

 season. A succession of five broods is not an unusual 

 number, and though the insect is not by any means what 

 might be called a prolific pest when compared with most 

 others, rarely laying more than one egg per day, still there 

 are considerable possibilities of multiplication involved in 

 this succession of broods. Large numbers of the insects 

 survive the winter, going into winter quarters in moss. 



