In this chapter we shall deal with a few aquatic insect larvae. Of these, some are aquatic also in the perfect condition, while others emerge from the pupa stage as aërial insects. It requires no great amount of care to keep these creatures, and some hints on this subject and on collecting are given in the first chapter.
The larva of Dytiscus is abundant during the greater part of the year, and is almost sure to be met with by the collector, who will find it an extremely interesting object for examination and study. No other creatures should be put in the same bottle with these larvae; and if there are several of them in one bottle, it is a good plan to put in plenty of pondweed, which will often keep them from attacking each other.
When full grown, the Dytiscus larva may attain a length of two inches or rather more. Its colour is dingy brown, and its aspect forbidding enough to justify the uncomplimentary names that have been bestowed upon it—Water-devil and Water-tiger. It certainly rivals the tiger in fierceness, and its method of stealing up to its prey and attacking it from behind led Swammerdam to call it the Sicarius or Assassin Worm.
One must not imagine that Swammerdam was ignorant of its nature; ‘worm’ with him was a general term for any larval form. Indeed, he says, ‘It is extremely probable that some peculiar species of the Water Beetle proceeds from this worm, when, having remained in the water a sufficient time, it betakes itself to the land to undergo its mutation; but this is mere conjecture.’ What was conjecture for him is fact for us.
Now let us put our larva into a small tube, and examine it more closely. The head is large and joined to the first segment of the thorax by a distinct neck. There are twelve small eyes, six on each side, a pair of antennae, two pairs of palps, and a large pair of sickle-shaped mandibles, which Swammerdam calls ‘teeth,’ and says that ‘it is perhaps to contain the muscles such teeth require that Nature has made the head so large.’ Behind the head come eleven segments, of which the first and last are the longest. They gradually increase in width till the sixth, the rest again decreasing, till the eleventh ends in a blunt point, from which diverge two appendages, thus
, thickly fringed on both sides with hair, as are the tenth and eleventh segments.
There are six legs, one pair to each of the first three segments. These also carry fringes of hair, thus increasing their power as swimming organs; and, in addition, they bear numerous spines, and end in strong double claws, which must be of service in climbing over aquatic vegetation, and may assist in holding a struggling victim or in striking it down, so as to bring it within reach of the mandibles.
Spiracles will be found—seven on each side. These do not, however, in the larval condition, serve as breathing organs, though they fulfil their proper office in the pupa. The air-tubes of the larva open at the extremity of the last segment. When the larva wants to breathe it comes to the surface without an effort, for it is lighter than the water it displaces. The tail rises above the surface, and a fresh supply of air is taken in. When the larva wishes to descend, a stroke of the tail sends it downwards, and as it reaches the bottom of the tube it will cling with its claws to any weed we may have put in with it, or hold on with them to the glass itself.