No distinction between head, thorax and abdomen can be observed.
Among the burrowing and tubicolous forms it is not uncommon for the body to be distinguishable into two or more regions; a "thorax," for example, is sharply marked off from an "abdomen" in the Sabellids.
In these forms the bundles of setae are either capilliform or uncinate, and the dorsal setae of the thorax are like the ventral setae of the abdomen.
In the adult state no insect possesses more than six legs, and they are always attached to the thorax; in many Thysanura there are, however, processes on the abdomen that, as to their position, are similar to legs.
The latter again are divided into Metanopsilae (in which the metanotum or posterior region of the thorax is bare) and Metanotrichae (in which the metanoturn is clothed with bristles or scales).
The insects of this order have mandibles adapted for biting, and two pairs of membranous wings are usually present; the first abdominal segment (propodeum) becomes closely associated with the fore-body (thorax), of which it appears to form a part.
A true insect, or member of the class Hexapoda, may be known by the grouping of its body-segments in three distinct regions - a head, a thorax and an abdomen - each of which consists of a definite number of segments.
The head is usually connected with the thorax by a distinct membranous neck, strengthened in the more generalized orders with small chitinous plates (cervical sclerites).
In addition to the wings there are smaller dorsal outgrowths of the thorax in many insects.
Other species of wandering habits carry the cocoon about with them, sometimes attached to the spinnerets, as in the Lycosidae, sometimes tucked under the thorax, as in the large tropical house-spider, Heteropoda regia, one of the Clubionidae.
Laennec, to whom we are indebted for the practice of auscultation, freely admits that the idea was suggested to him by study of Hippocrates, who, treating of the presence of morbid fluids in the thorax, gives very particular directions, by 1 " Hippocrates Cous, primus quidem ex omnibus memoria dignus, ab studio sapientiae disciplinam hanc separavit, vir et arte et facundia insignis " (Celsus, De medicina).
But the most natural division is obtained by the separation of the saw-flies as a primitive sub-order, characterized by the imperfect union of the first abdominal segment with the thorax, and by the broad base of the abdomen, so that there is no median constriction or " waist," and by the presence of thoracic legs - usually also of abdominal pro-legs - in the larva.
This sub-order, characterized by the " sessile," broad-based abdomen, whose fist segment is imperfectly united with the thorax, and by the usually caterpillar-like larvae with legs, includes the various groups of saw-flies.
Into a single nerve-centre situated in the thorax is remarkable.
Many of them, like ungulates, are specialized for swift running, and have unusually long limbs, with ridges developed on the articular surfaces of the lower bones; the clavicles are more or less reduced; the thorax is more compressed than usual, with a narrower breast-bone; and there is a marked tendency to the reduction or loss of the lateral toes, more especially in the hind limb.
The body proper is usually divisible into two regions to which the names thorax and abdomen are applied.
The nervous system is remarkably concentrated in some beetles, the abdominal ganglia showing a tendency to become shifted forward and crowded together, and in certain chafers all the thoracic and abdominal ganglia are fused into a single nervecentre situated in the thorax, - a degree of specialization only matched in the insectan class among the Hemiptera and some muscid flies.
- This is an important tribe of beetles, including families with four malpighian tubes and only five or six abdominal sterna, while in the thorax there is a backwardly directed process of the prosternum that fits into a mesosternal cavity.
The larvae are remarkable for their small head, very broad thorax, with reduced legs, and narrow elongate abdomen.
In colour Anopheles is usually brownish or slaty, but sometimes buff, and the thorax frequently has a dark stripe on each side.
A thorax also is sometimes to be distinguished from an abdomen.
- Tube-dwelling with body divided into thorax and abdomen marked by the setae, which are reversed in position in the neuropodium and notopodium respectively in the two regions.
Parapod.ia hardly projecting; palps of prosomium forming branched gills; no pharynx or eversible buccal region; no septa in thorax, septa in abdomen regularly disposed.
The thorax is composed of three segments; each bears a pair of jointed legs, and in the vast majority of insects the two hindmost bear each a pair of wings.
- Thorax of Saw-Fly (P I, Dorsal view.
Insects a remark - able concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax.
He finds that the endoderm arises may be readily distinguished, six of which subsequently enter into from an anterior and a posterior rudiment derived from the " endothe formation of the head, three going to the thorax and twelve to blast," that many of the cells of these rudiments wander into the the abdomen.
Lastly, the males of some species of spiders differ from the females in possessing stridulating organs consisting of horny ridges and spikes and lodged either between the mandible and palpus as in some species allied to Linyphia, one of the Argyopidae, or between the cephalo-thorax and abdomen as in Steatoda, one of the Theridiidae and Cambridgea, one of the Agalenidae.
In the thorax the pronotum and prosternum are closely associated with the mesothorax, but the pleura of the prothorax are usually shifted far forwards, so that the forelegs are inserted just behind the head.
Next the larvae make their way into the connective tissue in the pro-thorax, and ultimately bore a channel into the base of the piercing apparatus and come to rest between the hypopharynx and the labium.
The other structural characters of the Order may be briefly summarized as: - mouth-parts adapted for piercing and sucking, or for suction alone, and consisting of a proboscis formed of the labium, and enclosing modifications of the other usual parts of the mouth, some of which, however, may be wanting; a thorax fused into a single mass; and legs with five-jointed tarsi.
In the various groups of the Entomostraca, on the other hand, the terms thorax and abdomen, though conveniently employed for purposes of systematic description, do not imply any homology with the regions so named in the Malacostraca.
In one of these (Heteronotus trinodosus), the dorsal area of the forepart of the thorax is developed into a plate which projects backwards over the body of the insect, which retains its normal form, and conceals all but the head, wings and, legs.
The legs and lower part of the body are dark coloured, but the dorsal surface of the thorax and abdomen is coloured green and is raised so as to form a crest with jagged edges exactly reproducing the irregular margin of a fragment of leaf cut out by the mandibles of the ant.
All insects have the same regional division of the body into head, thorax and abdomen, the same number of legs, a pair of antennae and a segmented abdomen.
The coloration of tsetse-flies is sombre and inconspicuous; the brownish or greyish-brown thorax usually exhibits darker longitudinal markings, and when the insect is at rest the abdomen or hinder half of the body is entirely concealed by the brownish wings.
- Drawing from life of the Italian scorpion Euscorpius italicus, Herbst, holding a blue-bottle fly with its left chela, and carefully piercing it between head and thorax with its sting.
The segmentation of the prosoma and the form of the appendages bear a homoplastic similarity to the head, pro-, meso-, and meta-thorax of a Hexapod with mandibles, maxillary palps and three pairs of walking legs; while the opistho io i e d c b o a S' S" 2 I VT V S IV III II I Opisthosoma Prosoma FIG.
The dorsal skeletal elements of the thorax and of the anterior six abdominal segments unite with the wing-cases to form a large respiratory chamber, containing five pairs of tracheal gills, with lateral slits for the inflow and a posterior orifice for the outflow of water.
When the aquatic insect has reached its full growth it emerges from the water or seeks its surface; the thorax splits down the back and the winged form appears.
Size compared with the thorax or abdomen, but in the embryo it On the whole it seems likely that the endoderm is represented in forms a much larger portion of the body than it does in the adult.
Bristles are usually present on the legs, and in the case of many families on the body also; those on the head and thorax are of great importance in classification.