We know that all animals are specially adapted to the life which they have to lead, and therefore may naturally expect that Beetles which live in the water will be formed very differently from those which reside on the land, even though that land be constantly wet. Shape, for example, is likely to be altered. We know that the whales, dolphins, and seals, who have to pass either the whole or the greater part of their lives in the water, and to catch in it the living prey on which they feed, become assimilated in shape to the fishes ; and it is likely that insects will obey the same laws as mammals. This is really the case, the shape of all the Hydradephaga being very fish-bke, in order to enable them to pass more easily through the water. As there is much more friction in passing through the water than through the air, the Water Beetles, as the Hydradephaga are familiarly called, have the various portions of the body fitting closely to each other, so as to leave an uniform smooth and polished surface, something like that of the scaly surface of the fish, the slippery skin of the whales and dolphins, and the close-set fur of the seals.

The limbs are also modified to suit the special purpose for which they are designed. As these Beetles walk less than they swim, greater provision has to be made for the latter mode of progression. Accordingly, the first and middle pairs of legs are comparatively small and feeble, the strength being thrown into the hinder pair, which are large in comparison with the

others, and, in nearly all cases, flattened and furnished with a fringe of stiff bristles on the inner side, so that they serve as I oars. They are jointed in a peculiar manner to the body, so that there is room within the thorax for a set of very powerful muscles which work them, and they are placed farther back than is usual among Beetles — a peculiarity of structure which is found also in the seals and the diving birds, especially the penguin tribe.

Although they cannot, as a rule, walk well, they can all fly well, and are fm-nished with very large and powerful wings, so that, if food should fail them in one piece of water, they can fly to another. They generally fly at night, and have an odd way, when they reach a pond or stream, of closing their wings while high in the air, and allowing themselves to fall like stones into the water. Sometimes, deceived by the glitter in the moonshine, they have been known to fall upon the roofs of greenhouses.

Not only the Beetles, but their larvae inhabit the water, and they are equally predacious in both stages of existence, the larva being armed with a pair of enormous sickle-shaped jaws. They are all long and narrow, and have six minute eyes, or ocelli, at each side of the head. We will now proceed to our examples of these insects.

The Hydradephaga are divided into two families, the Dyticid® ' and the Gyrinidse. There is not the least difficulty in de- ' ciding the family to which any Water Beetle belongs, as a glance at the antennae is sufficient for the purpose. The antennae of the Dyticidae are long and slender, and those of the Gyrinidse are stout, short, and club-like. Moreover, the first pair of legs are short in the Dyticidse and long in the Gyrinidse. We begin with the fiirst family, and take an example of the typical genus.

On Plate III. Figs. 1 and 2, are seen drawings of the Great Water Beetle {Dyticus marginalis). The first represents the male Beetle in the act of swimming, and the second the female, as she appears when flying through the air. The sexes of this and other species are so distinct that in the earlier days of entomology they were looked upon as different species. As the habits of this Beetle are almost identical with those of all its

Plate Iii. Water Beetles.

Dyticus marginalis (Male). 4/ 2. Dyticus marginalis (Female). 8. Gyrinus natator (and under). 4. Dyticus marginalis, lai'va. 6. Gyrinus natator, larva. 6. Gyrinus natator, Pupa case (on Arrow-head). Plants ; — Arrow-head {Sagittaria sagittifolia). Left Upper Comer. Creeping Mouse-ear {Myosotis repens). Eight Upper Corner. Bristle-stalk Club-rush {Scirpus setaceus). Under Myosotis. Water Feather-foil {Hottonia palmtris). Under Arrow-head.

at greater length than can be af- , family, it will be described ( forded to the generality of insects. which mark the Beetles of this genus are

Several details No. 6. At Fig. a is a magnified representa- = given on Woodcut of structure. If one of I tion of a most curious development be examined on the under side, the metasteinum j these Beetles

Agabna biguttatus. 2. Hydrophilus dnodecim-pustulatus. 3. Ilaliplus vainegatus. 4. Cnemidotus csesus. 5. Pelobius Hermanni. a. Dyticus, process of metastemum. 6. Dyticiis, maxillary palpus. c. Dyticus, anterior leg of male. d. Dyticus, labial palpi, e. Gyrinus, posterior leg. /. Gyrinus, antenna.

■will be seen to have a forked and rather sharp projection from its centre, the points being directed to the end of the abdomen. What may be the object of this cirrious appendage is not easy to say ; certain, however, it is, that it can be used as a weapon on some occasions.

When, for example, an unpractised entomologist catches one of these Beetles in liis hand, and has taken care to keep his

fingers out of the way of its jaws, he finds himself suddenly and smartly wounded, as the Beetle struggles to regain its liberty. The fact is, the insect, led by some strange and unaccountable instinct, always retrogrades when seized in the hand, and so inflicts a rather unpleasant wound with the ends of this appendage. Whether or not it knows of the presence of the weapon, and the use to which it is put, is of course impossible to say ; but that the insect can use its forked dagger as well as if it were thoroughly acquainted with it, any of my readers can easily test for himself by going to the nearest pond and catching a Dyticus. Other Water Beetles possess the forked appendage ; but it takes different shapes in different species, and is exceedingly useful to entomologists, by enabling them to decide upon the species when other marks fail them.

This is, by the way, not the only weapon which the Dyticidse possess. Like the Carabidae, and some other Geodephaga, they exude a fluid of a singularly unpleasant smell when they are captured ; but the liquid in question is white, and not black like that of the Carabidae. At Fig. b is shown the maxilla and palpi of the Dyticus, and cl are the labial palpi.

As the two sexes are so dissimilar in appearance, it will be necessary to describe them separately. The colour of the male Beetle is dim black, with the margins of the elytra marked with a yellowish streak, narrow towards the apex, and widening considerably towards and on the shoulders. It is in consequence of this streak that the Beetle has received the specific name of marginalis. The elytra are very smooth, with the exception of three rows of punctures on the disc. There is a reddishyellow triangular mark on the forehead, and a very slight ridge on the crown'. The thorax is blacker than the elytra, and, like them, has the margin yellow.

The legs of this Beetle are excellent examples of these limbs as they are modified in the Hydradephaga. Both the middle and hind pairs of legs are flattened, oar-like, and fmnished with the bristle blade, and the coxa is so made that it only allows one kind of movement to the limb. In consequence of this peculiarity the Dyticus cannot walk properly, but only scrambles about ; and if it should by chance fall on its back on a smooth surface, it spins round and round in a most ludicrous fashion.

The first pair of legs, however, are the most interesting'. f We have already seen that, in very many Beetles, the tarsi of I the front pair of legs are dilated in the male, but there are 1 none which even approach those of the Dyticus in complexity i of structure. The geodephagous males have the under surface i of these dilated joints merely furnished with a pad, but the I Dyticus has a most wonderful array of suckers, exactly analogous in principle to those which stud the arms of the cuttlefish. One of these legs is shown on Woodcut No. VI. Fig. c. The three basal joints of the tarsus are enormously swollen, so that they assume a plate-like shape. Their upper svu'face is smooth enough, but the under surface is covered with suckers, one of them very large, and the second about half its size, and a multitude of smaller suckers. The larger suckers are placed directly upon the joint, and the others are at the end of slender footstalks, looking something like the ‘ patera ’ champagne glasses with the stems much attenuated.

In order to see this beautiful structure properly, several specimens ought to be used. One can be simply cleaned, and viewed as an opaque object, while another is cut so as to give a section of the joints, and to show the manner in which the suckers spring from them. Three more specimens should also be prepared, but mounted as transparent objects in Canada balsam. This will be found rather a difficult process, but one which will very well repay all the trouble. The difficulty is to get rid of the air which remains in the suckers, and which makes the mouth of each sucker look like a black shining ball. A structure, similar in principle, though varying in detail, is found throughout this group of Beetles.

The female is, as has been stated, very different from the male in appearance. She does not possess the wide tarsi which are so conspicuous in her mate. The elytra are very different from those of the male, and, instead of being smooth and polished, are deeply grooved nearly as far as the middle.

The voracity of this Beetle is really astonishing. It will eat almost any kind of insect, or any kind of meat, raw or cooked preferring the former. Sometimes it is placed in fresh-water aquaria by those who are not acquainted -with its powers, and the result is always disastrous. Not only will it attack every living creature in the aquarium, but during the night it

is sure to take to its wings and fly off in search of more food. Even gold fish have been attacked by this insatiate devourer, which does not even spare its own kind, and devours the opposite sex as well as its own. When these Beetles take flight from the pond or stream in which they have been living, they always crawl up a reed or a water-plant, so as to gain space to spread their beautiful wings. In consequence of this freedom of locomotion, this Beetle may be found in almost any pond, however small it may be. Even when ponds have been reduced to mere puddles by the drought of summer, the Dyticus may be found plentifully, hiding itself in the still soft mud.

The mode in which this insect breathes is really wonderful. Being an insect, it is forced to breathe atmospheric air, and yet it has to pass the greater part of its time under water. The problem is solved by the Beetle converting itself for the nonce into a diving bell, receiving the supply of air as often as needful. This feat is accomplished in the following way : — The body is rather flat, so that there is a space between the folded wings and the elytra. Now, these elytra are very large, and, when closed, are quite air-tight. When the Beetle wishes to breathe, it comes to the sm’face of the water with its head downwards, and just exposes the tip of its abdomen to the air. In a moment it has expelled the air which has already been used in breathing, and taken in a fresh supply, with which it dives to the bottom. As the spiracles, or mouths of the breathing tubes, open into the space between the elytra and the abdomen, they can take in the air, and pass it through the system. Sometimes, if the observer will approach very quietly, he may see the Beetles floating with their heads downwards, the tips of their tails just above the surface of the water, and their hind legs spread out so as to balance the body in this strange position. All the Dyticidae employ this cm-ious mode of supplying themselves with air, but it is most conspicuous in the larger species, and is therefore described in connection with this Beetle.

Page 13 of 117