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Little need be said as to the internal anatomy of Lithobius. The digestive tube is well developed, and is furnished with large salivary glands, and a rudimentary liver, together with certain tubes which represent the kidneys. The heart is in the form of a long, simple tube, placed along the middle of the back. The breathing-organs are in the form of delicate membranous tubes, the walls of which are strengthened by a spirally-coiled filament or fibre of horn. These tubes commence at the surface in little rounded apertures, one of which is placed on each side of each alternate segment, and they branch frequently as they proceed inwards amongst the various tissues of the body. The nervous system, lastly, has the form of a chain of pairs of little nervous masses, one pair being present in each ring or segment; these masses are united together so as to form a doubly-knotted cord, placed along the lower surface of the body.
Lithobius forficatus is a darkness-loving creature, and haunts obscure crevices of walls and cellars, or lives hidden under stones, or beneath the rotten bark of trees. It is highly carnivorous, and lives upon the bodies of small animals (such as earth-worms and caterpillars), which it kills by the bite of its empoisoned “foot-jaws." When captured, it will attempt to bite, but it is quite harmless, as the jaws are not sufficiently powerful to pierce the skin. Its general colour is brownish-red or ferruginous.
RECAPITULATION OF ESSENTIAL CHARACTERS.—Head distinct from the segments carrying the legs, and supporting a single pair of jointed feelers. Segments behind the head numerous and distinct, but not separated into regions. Legs numerous, usually from fifteen up to as many as one hundred and sixty pairs, sometimes eleven pairs, but never fewer than nine pairs. No wings. Breathing-organs in the form of branching tubes, adapted for breathing air directly. These characters distinguish the class of the Myriapoda as a whole.
THE true Insects derive their scientific as well as their ordinary name from the distinctness with which the body is cut or divided into distinct portions or regions (Latin, inseco, I cut into). Of the many excellent representatives of this class which might be selected, none, perhaps, is better than one of the larger Dragon-flies, such as the great Æshna grandis of Britain (fig. 17, A).
If we look at a Dragon-fly, we observe very readily that the body is more or less clearly divided into three portions, · a head in front, a chest (or “thorax") in the middle, and a tail (or “abdomen") behind. It will also be seen that the body is composed of a number of rings or "segments," which are placed one behind the other. These segments are very conspicuous in the tail or abdomen (fig. 17, A, a), are less conspicuous in the trunk or “ thorax," and cannot be clearly discerned at all in the head.
The entire skin is hardened with horny matter, so that each ring or segment forms a more or less resistant tube, within which the internal organs are protected.
Commencing with the head (fig. 17, A, h), no distinct rings can, as already remarked, be clearly made out; but nevertheless the head really consists of a certain number of segments consolidated into a single mass. Upon its sides, the head has two conspicuous shining globes (fig. 17, D, e e), which are the eyes. Each eye is what is termed “ compound," being really composed of an enormous number (several thousands) of minute eyes placed side by side, and doubtless conferring upon the creature a high power of vision. Besides these compound eyes, the head likewise carries three "simple" eyes, which are so minute as only to be visible with a magnifying glass. The head, further, carries upon its upper surface two jointed thread
like organs (fig. 17, D, an), which are called the “feelers" or “antennae” (Latin, antenna, the yard-arm of a ship),
Fig. 17.-A, One of the Dragon-flies (Æshna grandis), slightly dissected; h
Head, carrying the eyes, antennæ, and organs of the mouth; t, t', t' First, second, and third segments of the thorax slightly separated from one another, each carrying a pair of legs, and the two last carrying each a pair of wings; a Tail or abdomen. B, Young form, or “larva,” of the same. C, Second stage, or “pupa.” Ó, Head of a Dragon-fly (Libellula depressa), showing the feelers or antennæ (an), the eyes (e e), the hinder pair of jaws (m), and the upper lip (f).
and which no doubt are employed by the insect as organs of touch, and perhaps as organs of hearing as well. Upon its under surface, lastly, in front, the head carries the mouth, surrounded by the lips and jaws. These need not be particularly described, beyond saying that they
consist of an upper and lower lip, and of two pairs of strong jaws. The jaws do not work up and down, as in man, but from side to side, and they are adapted for biting, thus enabling the insect to live upon other insects, which it captures and devours.
Behind the head come three rings, which are slightly separated from one another in the illustration (fig. 17, A, t, t,'t"), but which in reality are consolidated with one another so completely, that they can only be made out by means of the appendages which they carry. They compose collectively a region of the body which is known as the chest or “thorax” (Greek, thorax, a breastplate). The first of these three rings (t) carries a pair of jointed legs; the second (t') carries another pair of similar legs, and a pair of wings; and the third (t'') carries a third pair of legs, and a second pair of wings. There are thus three pairs of legs and two pairs of wings.
The wings are really expansions of the skin, and are nearly of equal size. They are membranous, transparent, without hairs, and rendered gauzy by an extremely fine network of interlacing threads, which are known as “ nervures." These threads are really hollow ; and whilst they serve to support the fragile expansion of the wing, they also assist the insect in breathing, for they contain blood-vessels and prolongations of the breathing-tubes.
Behind the trunk or thorax comes the tail or “abdomen” (Latin, abdo, I conceal ; so called because it conceals the internal organs). This region of the body is very distinctly composed of separate rings (nine in number), none of which carry legs, and all of which except the last are devoid of any appendages at all.
Turning now to the internal anatomy of the Dragon-fly, a few words may be said about its organs of digestion, its nervous system, and its breathing and circulatory apparatus. The mouth, armed with its powerful jaws, opens into a gullet (fig. 18, 9), which conducts to a stomach (s). The stomach opens into an intestine (i), at the commencement of which are certain membranous tubes (1), which end in closed extremities, and which are believed to represent
either the liver or the kidneys. The intestine opens into a large chamber (c), which opens upon the surface by a distinct vent (v).
v C ¿ 1 f Fig. 18.-Diagram of the anatomy of an insect. an Antennae; e Eye ; m
Mouth ; g Gullet; sg Salivary gland; & Stomach ; f Tubes supposed to represent the liver ; i Intestine ; c Chamber into 'which the intestine opens ; v Vent; h Heart; n Nervous system ; 1 Bases of the legs.
The nervous system (fig. 18, n) consists of a chain of little nervous masses placed in pairs along the lower surface of the body, a pair of these masses being situated in each ring of the body. The first pair of nervous masses is placed above the gullet, and the second pair behind or below the gullet, and the cords which unite these two pairs pass on the sides of the gullet. It follows from this that the gullet is surrounded by a ring or “collar” of nervous matter.
The heart is in the form of a long tube (h), placed along the back, and furnished with flaps or valves, which only allow the blood to pass in one direction, namely, towards the head.
The breathing-organs are in the form of branched tubes, which commence on the surface of the body in little rounded openings, and then branch freely through the tissues, thus conducting the air to all parts of the body, and purifying the blood. The breathing-tubes are composed of a delicate membrane, the walls of which are supported by a horny fibre, which is coiled up in the interior in the form of a close spiral.
When young, the Dragon-fly is very different to the grown-up insect, and it passes through certain changes,