Leaving the crustaceans now, and passing for a moment to the Arachnoidea—the second great division of the arthropods—we shall briefly describe the Shore Spider (Pycnogonum littorale), which is the only representative of the class likely to be met with by the sea-shore collector.
It will be seen by our illustration that this creature by no means resembles a typical spider. The powerful jaws, really modified antennæ, that are such formidable weapons in the latter, together with other appendages of the head, are undeveloped in the shore spider, and the head is prolonged forward to form a rigid beak with the mouth at the summit, and the head and thorax together form a cephalothorax of four distinct segments, each of which bears a pair of legs. Further, the cephalothorax forms almost the whole of the body, for the abdomen, usually so large in spiders, is here represented by a mere tubercle. The shore spider is unable to swim, but crawls about among the weeds and stones of the bottom, clinging firmly by means of the curved claws of its eight thick legs, and is protected by its dull grey colour which closely resembles that of the encrusted stones among which it spends the greater portion of its existence. It may sometimes be found hiding under stones near low-water mark, but is far more commonly seen among the ‘rubbish’ hauled in by the trawl.
Fig. 219.—The Shore Spider
We shall conclude our brief survey of the marine arthropods by a short account of the insect life of the sea shore, referring to a few of the more prominent forms and observing some of their habits; but since it is probable that some of our readers are not well acquainted with the general characters of this interesting class of animal life, it will be advisable to precede our remarks by a short summary of their principal distinguishing features, more particularly those in which they differ from the other arthropods.
Insects, then, may be defined as those arthropods in which the body is divided into three distinct parts—the head, composed of from four to six fused segments, and bearing as many pairs of appendages; the thorax, formed of three segments, each of which gives attachment to a pair of legs; and the abdomen, composed of eight segments that bear no appendages.
The head of an insect is furnished with a pair of compound eyes, very similar in structure to those of a crustacean, and often, in addition, a cluster of simple eyes; also a pair of antennæ, usually composed of many joints. These antennæ are important organs of touch, and are employed, at least by many forms, as a means of communication between one insect and another. In them are also located the organs of hearing, and, possibly, those of other senses.
The mouth varies very considerably in different insects, but is often supplied with a pair of mandibles or biting jaws, and, below them, a pair of maxillæ or chewing jaws, both pairs being jointed to the head in such a manner as to be capable only of horizontal movements. Above and below these jaws are, respectively, the upper lip or labrum, and the lower lip or labium, the latter having appended to it a pair of jointed feelers called the labial palpi, and an additional pair of palpi are also frequently attached to the maxillæ, and therefore called the maxillary palpi.
These organs of the mouth of an insect are modified in various ways according to the functions they are called upon to perform. Thus, in bees, the upper lip, as well as the mandibles, are adapted for chewing, while the maxillæ and the labium are grooved in such a manner that when brought together they form a tube through which fluids may be sucked into the mouth. Also, in the butterfly and the moth, the maxillæ are not constructed for chewing, but consist of two channelled rods which, when approximated, form a long tube or proboscis employed for suction; and in these insects the labial palps are large for the protection of the proboscis, which is retracted and closely coiled between them when not in use. Further, in the bugs, the labium is long and tubular, while the mandibles and maxillæ are often modified into sharp, stiff bristles that work within the tube, the whole thus forming a combined piercing and sucking arrangement.