Fig. 81.—Fore-wing of Bee, showing marginal fold (× 7).
De Geer’s allusion to the wing of the wasp might have been extended to most of the Hymenoptera, as we may see by examining the fore and hind wings of a bee or a sawfly. Dr. Sharp[63] says, ‘The wings [of the Hymenoptera] are remarkable for the beautiful manner in which the hinder one is united to the anterior one, so that the two act in flight as a single organ. The hind wing is furnished with a series of hooks, and the hind margin of the front wing is curled over so that the hooks catch on to it. In some of the parasitic forms the wings ... have no hooks. The powers of flight, in these cases, are probably but small.’
If we were taking our subjects in consecutive order, the larva of Ptychoptera would properly come here, as being that of a Crane-fly. But since it will be convenient to examine another larva which resemble Limnobia in its breathing apparatus, we will take next the larva of Parap´onyx stratiota´ta, one of the China Marks, for it is extremely interesting and by no means hard to come by.
The China Marks are small moths, with white wings bearing dark markings, which have been supposed to resemble Chinese characters. Their larvae are aquatic in varying degree—that is, some breathe air in the usual way, by means of spiracles; while others, by means of gills, breathe the oxygen dissolved in the water.
Many collectors have, no doubt, taken these larvae, and cast them away in the belief that they were caddis-worms. Such was the experience of the Rev. Gregory Bateman, the author of Fresh-water Aquaria, who says (p. 259): ‘While hunting for fresh-water animals, one not seldom comes across an insect wrapped up in two or more green leaves, or pieces of leaves, of some aquatic plant. The leaves and the animal have somewhat the appearance of a caddis-worm in its case; in fact, for a time, before I knew what it was, I mistook it (and I daresay others have done so also) for a caddis-worm.’
The cases are usually, but not invariably, made from the food-plant of the insect. Mr. Bateman has noted that these larvae ‘do not always confine themselves to the same weed, either for food or for building material.’ This has also been my experience. A larva of the Brown China Mark, recently taken on the Norfolk Broads—an excellent collecting ground for aquatic larvae—was put into a tube. The case had been damaged, and the only vegetation in the tube was a spray of bladderwort. On examining the tube, after some days, I failed to find the larva. The reason was evident on removing the cork, a small part of which had been gnawed away to procure material for the repair of the larval case, which was affixed to the under side. The larva was dead and too much decayed to be put into pickle, a circumstance I much regret, as I should like to have preserved the larva in such a strange dwelling. As it is, I have only been able to keep the house without its tenant.
Pondweed is the usual home of the larva of the Brown China Mark, and from the leaves of this plant the first larval case is generally fashioned. This was the species upon which Réaumur made his interesting observations, most of which have been confirmed by succeeding observers. In well-grown larvae the contrivance by which the animal is protected from contact with the water in which it lives should be noted, as it may be easily, with the hand lens. The skin is thickly set with tiny protuberances between which the water cannot penetrate, the surface film stretching from tip to tip of these prominences, just as it does over the hairs that cover the body of a water-spider.
De Geer[64] describes an aquatic larva of one of the China Marks (Paraponyx stratiotata), which has its breathing apparatus fashioned on a similar plan to that of Limnobia, though there is some difference in the details. He found his specimens on the leaves of the Water Soldier, and his interesting account recalls the fact to memory that this remarkable plant was at one time called the Marsh Aloe—an exceedingly appropriate name.
He describes the filaments on the body of the larva, and concluded that they were probably breathing-organs, because of the dark-coloured vessels within them. These he traced, as we will presently do, to their union in the stem of the gill, and thence to the air-vessels running down each side of the body of the larva. He fed them on leaves of the Water Soldier, and kept them through the winter. In the following June he found them preparing to undergo their transformation into the pupal stage, and at the end of the month the moths came out. He was gratified by seeing the congress of these insects. The females deposited their eggs on the floating vegetation and on the sides of his aquarium, a little below the surface of the water, and in about eight days the young larvae were seen.