Sept. 12, 1884.] 



♦ KNOWLEDGE ♦ 



211 



stingers, and in Scotland go by the name of " devil's 

 darning-needles," and in America by that of " mosquito 

 hawks," will detain us later on, when we speak of the 

 fauna of the atrial regions just above the pond. Suffice 

 it here to say, that they have four large glassy-looking 

 wings, reticulated with a multitude of nervures, and 

 usually a long, slender body, which has suggested to 

 our Highland brethren the diabolical connection 

 above mentioned. For brilliancy of coloration they 

 easily take a prominent position in the insect 

 world ; but their beauty pertains wholly to the adult 

 form. In their aquatic stages they are the dingiest of 

 the dingy, and in many cases are hideously ugly. When 

 you have brought yourself to perform the disagreeable task 

 of hauling out of a dirty pond a mass of slimy weeds and 

 fetid mud, and have deposited it on the bank, you see the 

 mass here and there heaving with the struggles of these 

 ugly brutes as they gradually work their way into daylight 

 and drag their grimy bodies out of the tenacious and 

 unsavoury mess. What a contrast between this sordid 

 life and the gay and brilliant existence of the shiny- 

 winged adult, as it dashes about, glistening in the 

 sunbeams ! There are two principal types of these 

 larvre ; one a broad, thickset, clumsy creature, which 

 yields the larger and stouter - bodied dragon-flies, 

 the other slender and carrying some leaf-like ap- 

 pendages at the tail, the immature condition of the most 

 slender and graceful members of the group. Taking first 

 the former of these (Fig. 1), we see a creature with six 



Fig. 1. — Larva of Dragon Fly. 



straggling legs, which, sprawling out at the sides, would, 

 were it not for their number, be strongly suggestive of 

 affinities to the spider class. The head, when viewed 

 from above, is surprisingly like that of a kitten, the 

 prominent ears of the latter being represented by 

 the equally prominent eyes of the insect ; the two short 

 antennse, too, are suggestive of the kitten's whiskers. 

 Then comes the thorax, with curious ridges Like rough bark, 

 and carrying the six sprawling legs and the rudimentary 

 wings, and then the abdomen, broadest a little behind the 

 middle, and exhibiting, especially in its hinder part, periodi- 

 cal contractions and dilatations, the length being lessened 

 at the expense of the breadth. This motion, as might be 

 expected, is a respiratory one. The breathing is performed 

 in a manner as wonderful as it is unique. It is most 

 marvellous what a variety of contrivances there are to 

 enable aquatic insects to perform this important function ; 

 we have already referred to the diving-bell arrangement of 

 the water-beetles, the anal spiracle of some of their larva?, 

 and the lateral leaf-like appendages of others, the long tail- 

 filaments of the water scorpions, and the feathery stars and 

 tubes of the gnat larva^ and pupa?, and now we come to an 

 arrangement totally distinct from all of these. At the 

 extremity of the body there are some stout, spiue-lLke 

 processes, surrounding the terminal orifice of the diges- 

 tive tube, which is guarded by a valvular apparatus. By 

 mxiscular effort these spines, which are movable at their 



base, can be opened out like the parts of a wire egg- 

 whisk, the capacity of the abdomen being at the same 

 time increased ; the valves are thus opened, and water 

 rushes in and fills the terminal part of the intestinal 

 canal, and after remaining there a short time, is forcibly 

 ejected by a reversal of these operations. The lining of 

 the last part of the intestine is produced into six double 

 series of folds, whereby its surface is enormously increased. 

 In the interior of these thin folds, great numbers of minute 

 tracheal tubes are distributed. The water, of course, as 

 usual, contains air dissolved in it ; and, as it passes over 

 these tracheal tubes, the fresh air with which it is charged 

 can be exchanged for the contaminated supply contained in 

 the tubes, by simple transfusion of the gases through the 

 thin walls of these. As soon as this has been effected, 

 the now useless water is got rid of in the manner 

 above described, but its expulsion frequently serves the 

 additional purpose of effecting locomotion. Wlien the 

 insect is calm and undisturbed, the water is passed 

 out gently, but should it be disturbed or alarmed, a 

 forcible ejection of the liquid follows, and just as a rocket 

 mounts in the air while the gases into which its contents 

 are being transformed by the process of combustion, rush 

 out in the other direction, so the larval dragon-fly is shot 

 swiftly forward as the jet passes out from behind. The 

 jet can be readily observed : if there are particles of 

 matter in suspension in the water, their movements as 

 they are carried slong with the stream make the current 

 perceptible ; or if the creatures are in a shallow vessel 

 with only just enough water to cover them, the sur- 

 face will be seen to be violently disturbed at every expira- 

 tion. The force with which the water can be projected is 

 quite surprising ; the most astonishing record comes from 

 over the sea. A lady states in the American Naturalist 

 that a larva of a large species, when disturbed, sent out a 

 fine stream of water to the distance of from two to three 

 feet, and continued doing so indefinitely ! 



These curious beings then progress by a series of jerks 

 or leaps, though, of course, they can crawl as well. Some 

 kinds while jerking themselves forward, assist their efforts 

 by a sharp backward stroke of the legs (though these are 

 not modified for swimming purposes), and finish the stroke 

 by bringing the legs close alongside the body, an action by 

 no means inelegant. 



The more slender kinds have an elongated body, which 

 they can move pretty vigorously from side to side as a fish 

 does its tail. They have also three external leaf-like 

 appendages at the tail, which are thin, and are each sup- 

 plied with a tracheal tube and its branches, the exchange 

 of gases taking place here externally in the same way as in 

 the others internally. These caudal leaves, too, are used to 

 assist locomotion. 



But we have yet to consider one of the most remarkable 

 peculiarities of these creatures. If we take a front view of 

 the head, we see the lower part of the face rounded and 

 smooth, with a vertical zigzag line down the centre, and 

 showing no traces of great jaws such as one would expect 

 in so voracious an insect. This is simply because they are 

 concealed by a very curious modification of that part of 

 the mouth which, in insects generally, is called the labium, 

 or lower lip. To examine this structure it is best to take a 

 freshly-killed specimen ; this is easily obtained by plunging 

 the creature into boUiny water, which produces instanta- 

 neous death. By aid of a pin or needle we can now easily 

 open out the " mask," as it is called (Fig. 2), and when 

 fully extended, we see that it looks something like a 

 broad-handled ladle, attached by the handle underneath 

 the head. It consists of several joints, the basal one 

 of which is attached to the lower part of the head, 



