The Sea Shore

by William S. Furneaux

1. Æpophilus. 2. Machilis maritima. 3. Isotoma maritima. 4. Cœlopa

We frequently meet with a pretty, slender-bodied insect, measuring about half an inch in length without appendages, creeping over the rocks in the sunshine, generally very near the crevices in which they hide, and leaping from place to place when disturbed. These are the Bristle-tails (Machilis), belonging to the order Thysanura, the members of which, like the bugs, scarcely undergo any metamorphoses. This insect (fig. 222) has long antennæ, and also a long, stiff, and elastic bristle extending backwards from the tip of the abdomen; and this bristle is the means by which the creature leaps. Occasionally the machilis may be found resting on the surface of the still water of a rock pool, in which case its body is not wetted, its weight not being sufficient to break the surface film of the water; and, in fact, the film is even sufficiently firm to enable the insect to leap on the surface just as it would on a solid body.

Allied to the bristle-tails, and usually grouped with them in the same order, are the little Spring-tails, some species of which may often be seen huddled together on the surface of the water of a rock pool. They are so small that, unless closely examined, they may be mistaken for particles of floating inorganic matter which have been blown into a sheltered corner of the pool, and this idea may be strengthened by the fact that these minute creatures are driven by the wind into such sheltered spots. But when we disturb them their true nature immediately becomes apparent, for they may then be seen to move about on the surface of the water, sometimes creeping on the surface film, and clambering on the adjacent rock or weed, or leaping more or less vigorously, in which latter case their bodies do not become wetted, the surface film remaining unbroken by their exertions. And even when the rising tide drives the spring-tails into crevices where they remain submerged, perhaps for hours together, their bodies still remain dry, the water being kept off by numerous short bristles and prominences with which they are furnished.

When we examine a spring-tail by means of a lens we observe that it has no traces of wings, but that each of the three segments representing the thorax bears a pair of short legs, and that the abdomen consists of only five or six segments. The head is furnished with a pair of jaws, and the antennæ, which are short and thick, are composed of but few joints—never more than six in number.

Some spring-tails live among the refuse washed up on the beach, where they may be seen jumping about in company with the sandhoppers when the material is disturbed. Such is the case with Isotoma maritima, the illustration of which shows the forked tail that enables the little animal to jump about so vigorously. But some of the marine spring-tails are not so true to their name, since they are not provided with this characteristic jumping organ, and have to content themselves by creeping about slowly with the aid of their short legs. One of these springless spring-tails (Anurida maritima) is one of the commonest of the group, and is distributed over almost every part of our coast.

Passing over several orders of insects which do not seem to have any marine representatives, we come to the Diptera or two-winged insects, of which the familiar house-fly is a type, and here we have to deal with those troublesome creatures that literally swarm in the neighbourhood of the matter washed up to the highest level of the tide during the whole of the summer months. But although these insects are so very numerous, we do not find among them a particularly large number of species, their abundance being due more to the extreme prolificacy of those that occur.

In this order, which includes all gnat-like creatures, as well as those insects that are generally known as flies, the first pair of wings are well developed, while the second pair are rudimentary, and represented merely by a pair of scales, or by two little pin-like bodies called the balancers or halteres. Some are provided with piercing organs by means of which they can inflict a small wound and then extract the juices of their victim, as does the female gnat, but the majority have a proboscis adapted for suction only. The larvæ of the Diptera are generally limbless maggots, gifted with a pair of jaws, and they are usually very voracious feeders, devouring decomposing animal or vegetable matter in enormous quantities.

If we turn over a fermenting mass of the miscellaneous matter thrown up on the beach quite beyond the reach of the tides, we may observe a multitude of little maggots which feed on the moist, odorous portion that was protected from the direct rays of the sun, together with a number of dark-coloured pupæ that lie at the very bottom of the heap or buried in the sand below. These are two stages of the black fly (Cœlopa frigida) that is so attentive to us when we rest on the dry sand above high-water mark. This fly is very like the common house-fly in general appearance, though its body is rather smaller. Other species of the same genus often accompany them, all being very similar in general appearance and habits, and none of the larvæ seem adapted to a life in the water. They are always found beyond the reach of the tide, and are drowned if submerged for any length of time.

Another species belonging to the genus Actora will often be seen in the same company, and this is readily distinguished by their lighter greyish colour and its superior size. Also, along the water-line, we often meet with species of the family Dolichopodidæ, so called on account of the length of their legs, and noted for the beautiful metallic colours which adorn their bodies. These flies are carnivorous in habit, deriving their food from living as well as from freshly killed animals, and their short, fleshy proboscis contains a piercing bristle by which they can puncture the skins of the animals that provide them with food. Most of the flies of this group live on trees, walls, fences, &c., where they pursue and attack their prey, but certain species follow the line of breakers on the sea shore, as before indicated, and obtain their food from the various marine animals that are stranded on the beach. A peculiar feature of the family is the nature of the abdomen of the males, which is bent under the body and furnished with a number of appendages.

Another marine dipterous insect is a gnat-like fly closely allied to Chironomus, which we have described in a former work[*] of this series dealing with fresh-water life; and it will be sufficient to mention here that Chironomus is commonly known as the window-gnat on account of the frequency with which it may be seen flying on the windows of our dwellings; also that the larva, known popularly as the bloodworm, is truly aquatic in habit, being able to swim by rapidly looping its body in opposite directions, and being provided with a breathing apparatus adapted for the absorption of the oxygen gas contained in solution in water. The larva of the marine species referred to above may sometimes be seen in rock pools, where it shelters itself among the sediment at the bottom. It is much like the bloodworm in appearance and structure, but its body is greenish instead of red.