SCIENCE- GOSSIP. 



291 



rents my at present dwelling further on the sub- 

 ject. 



The larvae of the Dytisci have not inaptly been 

 called " water-tigers," and were considered by early 

 naturalists (Mouffet, etc.) to be closely allied to 

 the shrimp; thus for a time we find them included 

 amongst the crustaceans, where in the genus Squilla 

 they figured as 8. aquatica. They commence to 

 feed shortly after birth, and, like the young of the 

 generality of Coleopterous carnivora, soon attain 

 their greatest size (fig. 1). During growth the skin 

 is repeatedly cast, but I am unaware of the exact 

 number of ecdyses effected. In from a month to 

 six weeks the larvae become full grown, at the ex- 

 piration of which period they often measure as 

 much as two inches in length. One in my posses- 

 sion taken at Hoylake during the summer of 1899 

 exceeds 58 mm., or a fraction over two and a quar- 

 ter inches long. They may be captured, in various 

 stages of growth, in similar situations to the perfect 

 insect, and can be kept alive and fed in the same 

 way, so that their movements in the water are 

 easily studied. The general form may be said to 

 be spindle-shaped, something like a torpedo with a 

 head and legs. Like a torpedo on occasion, too, 

 the larva is equally capable of rendering itself 

 eminently disagreeable to the various dwellers in 

 its immediate vicinity; for its appetite equals, if 

 possible, that of the adult insect, which is saying 

 much, whilst its energy is untiring. 



The large, flat, circular head is armed with strong 

 sickle-shaped jaws, traversed throughout their 

 length by hollow tubes, so that when the insect 

 has fastened them upon its prey it is enabled to 

 suck up the juices of the unfortunate victim. 

 Many authors state that this is the larva's only 

 mode of feeding (Siebold, etc.), and Nagel was of 

 opinion that the method was rendered easier by 

 the injection into the body of its captive of a kind 

 ■of " digestive fluid," which rapidly dissolved all the 

 more solid parts of its organisation and thus pre- 

 pared them for assimilation. These theories can 

 scarcely claim the support of modern investigators. 

 Besearches by De Geer, Meinert, Burgess, Miall, 

 and others have served to show that in addition to 

 the mandibular perforations the larva is possessed 

 of a small but true mouth, by which it is enabled 

 to consume minute portions of solid food. The 

 head is joined to the body by a distinct neck, and, 

 after gradually increasing in size until near the 

 middle, the posterior portion of the insect tapers 

 to the apex, where the extremity of the eighth 

 abdominal segment terminates in two short cerci, 

 which are prominently supplied with hairs. These 

 appendages, in addition to being used on occasion 

 as auxiliary organs of propulsion, further serve to 

 sustain the larva at the surface of the water during 

 respiration. The legs are long, the rows of stiff 

 hairs running along their inner surfaces well fitting. 

 them for swimming purposes, whilst the cruel 

 double tarsal hooks are used in the capture and 



tearing of its prey. The general colour of the larva 

 is of a dirty greyish-brown tint, thus usually 

 harmonising with the muddy bottoms of the pools 

 and sluggish streams in which the larva lies in wait 

 for its victims. 



The period spent in the larval state probably 

 varies with the season of the year ; those hatched 

 out during the early summer may attain their full 

 size in from fifteen (Fowler) to twenty days, whilst 

 others, emerging later, bury themselves in the 

 muddy banks of their pools should cold weather 

 set in, pupating only at the approach of spring 

 (Miall). 



As the time draws near for its transformation 

 the larva crawls out of the water and constructs a 

 rude cell in the ground close by, where it under- 

 goes its change to the pupal state. This inter- 

 mediate change is always terrestrial, and lasts about 

 three weeks or so, at the expiration of which period 

 the perfect form is reached. 



The newly emerged imago is at first quite soft 

 and of a light testaceous hue, but gradually hardens 

 and darkens, until at the end of a week it presents 

 pretty much the general colour and solidity with 

 which we are familiar. In its perfect state Dytiscvx 

 marg ma lis again resumes its aquatic existence, and, 

 once more adopting its piratical life, no insect or 

 other creature capable of being overcome is safe 

 from its ferocious attack. Having enormous 

 appetites, the beetles are often very destructive 

 to young fish ; Frank Buckland, amongst others, 

 having called attention to the damage wrought by 

 them to young salmon in Galway during 18(35 

 (•• Field," November 26th). In writing of CyMster 

 roeseli, a closely allied species, at one time in- 

 cluded in our British list, Burmeister records that 

 in forty hours one of his captive insects ate and 

 digested two frogs. 



In then broad structural features the Dytiscidae 

 resemble the Geodephaga, or predaceous ground 

 beetles ; but in the same marvellous manner that 

 the general form of the Cicindela is especially 

 suited for speed on land, is that of the Dytiscux 

 eminently adapted for rapidity of movement in the 

 water. In shape it is oval and somewhat flattened, 

 the broad head fitting deeply into the pro-thorax, 

 which in turn closely adjusts itself to the base 

 of the elytra, so that a continuous outline is im- 

 parted to the body as a whole. Having, moreover. 

 a smooth polished surface, more particularly in the 

 male sex. and being without pubescence of any 

 kind, there is nothing to exert any retarding in- 

 fluence as the beetle passes swiftly through the 

 water. The 11-jointed antennae are smooth and 

 glabrous, devoid of hairs, and inserted imme- 

 diately in front of the large compound eyes. The 

 jaws arecurved;as in the terrestrial Adephaga. and 

 are now solid and much stronger than in its larval 

 state, as becoming an insect whose whole muscular 

 system has considerably developed since its pre- 

 pupal days. 



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