In speaking of peculiar water insects one must not forget to mention the larva of Donacia. The adult female of this interesting leaf-eating beetle often cuts circular holes in the large leaves of water-lilies, and then deposits her eggs at the margin of these holes on the under side. When the larvæ hatch they make their way to the roots, upon which they feed. The really remarkable thing about this larva is how it gets its air supply, as it does not have gills, nor is it known to visit the surface for a fresh supply of air, and yet it has a normal air-breathing system. On the dorsal surface, near the tail end of the body, are two slender, curved, spine-like processes. The air tubes of the body arise from the base of these spines, and spiracular-like openings are found at their base.
Two different views have been advocated to explain how it is possible for the larva to secure air. There seems to be no difference of opinion with regard to the source of the air supply, from the air cells in the root of the plant upon which the larva feeds. One view is that these air spaces in the plant are punctured by the spines and thus the air is taken directly into the air tubes. The other view is that the larva bites a hole into the air space and then, by the aid of the spines, holds the openings at the base of the spine against the air space and thus the air is taken up.
The Back-swimmers are curious little fellows which swim upside down in the water, and by means of their sucking mouth parts, prey upon other small animals. The lightness of their bodies and the large amount of air which they carry with them make it necessary when they wish to remain below the surface to hold fast to some object. Thus it takes constant effort in order to remain below the surface. For this reason it is quite natural that they should very often be found floating at the surface where no effort is needed to maintain their position and where an abundant supply of air is accessible.
Another method of securing fresh air, but differing from that of any of the insects previously mentioned, is by means of elongated breathing tubes, thus allowing the insect to remain submerged and yet secure a fresh supply of air from the surface. This method is used by some predaceous water bugs, as shown in the plate, Ranatra fusca. As with all of our Predaceous water bugs, which have the elongated respiratory tube at the end of the body, the Water-scorpion has its fore legs adapted for capturing and holding its prey, which consists generally of small fish and insects. The apical part of the fore leg folds back on the basal part which is grooved on its inner face, as a knife blade folds into its handle. As the slender legs of this bug would indicate, it is not an active swimmer, but crawls about slowly.
Doubtless the best known, to most people, of this type of breather, are the Giant water bugs, which accumulate in such large numbers under and in the globes of electric lights. The paired nature of the breathing tube is well shown in the plate. These bugs are powerful swimmers, as is shown by their flattened hind legs. Even young fish are not overlooked by these voracious bugs. A South American kind is much larger than our species, reaching from four to four and one-half inches in length, or about twice the size of our species. The shortness of the air tube suggests that this organ is not used in just the same manner as in the Water-scorpions, and the areas of fine hairs on the under side of the body suggest that these insects may be somewhat of a compromise between those insects which carry air below with them and those which remain submerged, except for the tube which communicates with the air.
Most of the insects previously mentioned are ones which throughout life live in water, but a very large number are aquatic only during their larval or immature stages. The Mosquito is a good illustration, of this type. In some of its habits the Mosquito is well known, but this is primarily due to the biting habit of the female. The researches of recent years clearly show that the annoyance from the bite itself, is, in the case of some kinds of Mosquitoes, only a small part of the mischief that they can do. The life history of the Mosquito has been summarized somewhat as follows by Dr. L. O. Howard: The eggs are laid at night, in a boat-shaped mass containing from two hundred to four hundred eggs. These may hatch in 16 hours, the larval stage lasting about a week, and the pupal stage about 24 hours. Thus the entire cycle may be completed in 10 days, under favorable circumstances, but may be greatly delayed by a low temperature. The rapidity with which the complete cycle may be passed through makes it possible for a very large number of broods to occur during a single season.
The Wigglers or Wiggle-tails, often so numerous in rain-barrels, are the larvæ of mosquitoes. Every one has noticed that these larvæ when not disturbed rest at the surface, but when frightened drop slowly downward in the water, since they are heavier than this medium, yet they rest at the surface, by means of a rosette of thin plates at the tail end of the body. These act as the hairs on the legs of the Water-strider, and make use of the tension of the surface film which holds the larva up, as the surface tension held up the Water-strider. On the next to the last segment of the Wiggler there is a large breathing tube which reaches to the surface when the larva is floating. The food of the larva is said to be decaying vegetable matter. The short pupal period is also passed floating, but it now has two breathing tubes near the points of attachment of the wings. When ready to transform it crawls out onto the pupal skin and dries its wings preparatory to flight.
Our common mosquitoes belong to three genera, Culex, Anopheles and Corethra. The annoyance caused by the irritation resulting from the bite is not understood, as no poison gland has been found. The females only of our mosquitoes are known to suck blood. From researches made during the past few years it is now definitely known that the bite of certain kinds of mosquitoes is really dangerous. This is not on account of the actual puncture made by the insect, but due to the presence of the germs of malaria, which are introduced into the wound from the infected insect. The only mosquitoes which are definitely known to transmit this malarial parasite to man belong to the genus Anopheles. The malarial parasite thus has two hosts, mosquitoes belonging to the genus Anopheles and man. This parasite infests the stomach walls of the mosquito, where it rapidly multiplies and becomes mature; then escaping from this locality, accumulates in the salivary glands. From this reservoir they are easily transferred to their human host at the time of sucking blood.
These aquatic insects which we have discussed so briefly are only a few samples from a very large number whose history and habits are full of interest to those who find the study of animal life a fascinating subject.