Popular Science Monthly/Volume 45/August 1894/A Family of Water Kings
By Prof. CLARENCE M. WEED.
THERE is, perhaps, no way in which one can obtain a more vivid idea of the intensity of the struggle for existence among organic beings than by the study of the inhabitants of a freshwater pond of long standing. Every inch of space in such a situation is teeming with life, both animal and vegetable, and the chief delight of most of the animals present is to wage a ceaseless warfare upon their weaker fellows. It is an aquatic rendition of Edwin Arnold's aërial drama:
"... Then marked he, too,
The largest insects occurring in our fresh-water ponds are the giant water bugs—a family of peculiar creatures, armed with immense front legs fitted for grasping and clasping their victims, and a piercing, dagger-like beak which serves both to strike the prey and as a sucking tube to extract its juices, and which also appears to be provided with poison glands which make more sure the effect of every thrust.
Three species of these bugs occur in the Northern United States. Two of them are very large and closely resemble each other; the third is much smaller, less than half the size of the others. The commoner of the larger ones in the more northern States is represented natural size in Fig. 1. It is called by entomologists Belostoma americana, or the American belostoma. It is brown in color, with leathery wings overlapping each other on its back; thick legs, along the sides of which are fringes for swimming; and a flat, boat-shaped body which offers little resistance to the water.
The eggs of the American belostoma are deposited on pieces of wood or reeds along the margins of ponds, apparently where Fig. 1.—American Belostoma. they will be wet but not directly in the water. They are laid in clusters of from forty to sixty or more in each. The eggs themselves are about one fifth of an inch long, oblong-ovate in form, with the general color brown spotted with black; they are lighter colored below than above, and there is a whitish crescent near the top with a distinct black spot in its apex. This crescent indicates the margins of a little cap which conies off when the young bugs hatch.
Little seems to be known concerning the early history of these They probably crawl into the water soon after hatching, and live upon such aquatic insects as they are able to catch. I do not know just how fast they grow, but presume they become full grown in a year. During the earlier period of their existence they have no wings; they are then in what the naturalists call the nymph state. Their appearance just before they become adult is represented in Fig. 2. It will be seen that they have no Fig. 2.—Last Stage of nymph. wings, but otherwise they very much resemble the full-grown bugs. Finally, the skin splits open along the middle of the back, and the insect crawls out of its old skin clothed in a new one which is provided with wings. It now for the first time can leave the pond where it has developed, and fly away to other bodies of water.
If the front leg of a full-grown American belostoma be examined carefully, there will be found on the front margin of the long joint nearest the body a longitudinal groove for the reception of the next joint. By this character the present species can always be distinguished from the other one, in which there is no groove. This latter insect is called Belostoma griseus. It is usually a little larger and darker colored than its American cousin, and apparently is more common southward than at the North, The life histories of the two species, so far as we know them, appear Fig. 3.—Eggs. to be very similar. The eggs of the southern form are laid in masses on sticks or other rubbish at the margins of ponds. The general color of recently laid specimens is greenish brown, with longitudinal stripes of darker brown, and a faint indication of a light crescent near the top. Their bases are glued to each other and to the stick on which they are deposited by a sort of mucilage. An idea of the appearance of these eggs may be obtained from Fig. 3.
In South America a still larger species is found; it is called Belostoma grande, or the great belostoma. Still other species occur in Central America and Cuba, China and India, Egypt and Africa, but none are found in northern Europe.
Wherever these bugs appear they are formidable enemies of small fishes, frogs, and other aquatic animals. Of the Belostoma griseus, Prof. Uhler writes: "Developing in the quiet pools, secreting itself beneath stones or rubbish, it watches the approach of a mud-minnow, frog, or other small-sized tenant of the water, when it darts with sudden rapidity upon its unprepared victim, grasps the creature with its strong, clasping fore legs, plunges its deadly beak deep into the flesh, and proceeds with the utmost coolness leisurely to suck its blood. A copious supply of saliva is poured into the wound, and no doubt aids in producing the paralysis which so speedily follows its puncture in small creatures." In the breeding ponds of the Massachusetts Fish Commissioners Fig. 4.—River Zaitha. these bugs destroyed so many young fish a few years ago that the authorities had to take special pains to catch and kill them.
In many localities these insects have lately received the popular name electric-light bugs, because they fly so freely to electric lights. This indicates that in going from pond to pond they are nocturnal.
There is another species belonging to this family which is common throughout most of the United States. It is less than half the size of those we have been discussing, and is called by entomologists Zaitha fluminea, or the river zaitha; and is also known as the lesser water bug. It is a brown insect of the size and shape shown in Fig. 4. Its legs are provided with fringes for swimming, and it has a slender, sharp-pointed beak. As one would expect, it feeds on smaller animals than do the belostomas. A few years ago I dredged a number of these bugs out of an Ohio pond, together with a great quantity of other forms of pond life, and placed them all in glass aquaria to study their feeding habits. The bugs seemed to feed most voraciously upon the larvæ or Fig. 5.—Undulating Backswimmer. nymphs of dragon flies. These were captured continually, and their juices greedily sucked out. The next most abundant victim was the common undulating backswimmer (Notonecta undulata) shown in Fig. 5. In one aquarium, in which a large amount of pond material, including half a dozen zaithas, had been placed, four of the latter were in sight at one time, each with one of these backswimmers grasped in its front legs and the beak inserted in the body.
Small fresh-water snails occasionally contribute to the diet of this insatiable creature, and young mayflies are also commonly eaten. Flying insects which fall upon the surface of the water are sometimes caught and killed.
The giant water bugs are typical examples of the true bugs. They belong to the group called by naturalists Heteroptera, the members of which are characterized by having two pairs of wings, the front pair being thickened at the base and thin at the tip, and mouth parts fitted for sucking rather than for biting. During their development they do not undergo so complete a series of changes as do the caterpillars, which transform into butterflies, but grow more like the grasshoppers, the young resembling the adults in general appearance but having no wings. These bugs can be dipped out of ponds and ditches almost anywhere by means of a net, and are easily kept in aquariums, where they form interesting objects for study.
Illustrating, in one of his juvenile lectures, the liquefaction and solidification of gases, Prof. Dewar said that ether is evaporated to produce, by abstraction of heat from the gas, solid carbonic acid, which, though a white substance like snow, is boiling at 80° C. below zero. If the pressure is reduced by the air-pump, it boils at a lower temperature, and -110° C. may thus be reached. This is sufficient to liquefy nitrous oxide, which boils at -90° C.; and liquid nitrous oxide under the air-pump produces cold enough to liquefy ethylene, which boils at -100° C. The last stage is to liquefy air under pressure by the cold made by evaporating ethylene. In practice all these stages are not used, but they illustrate the gradational method which must be employed. The lecture was illustrated by liquid air being handed round in a flask inclosed in a vacuum jacket; though at a temperature of -180° C, it was boiling gently away. An idea was given of the difference between its temperature and that of the room by dropping it on a cold metal plate, when it assumed the spheroidal state like water on hot iron.