INSECTS. 



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INSECTS. 



of a number of somewhat angular lobes con- 

 nected by narrow processes having inter- 

 spaces between them. These are originally 

 formed from rounded nucleated cells, which 

 have given off' anastomosing processes (fig. 

 29). It is traversed by a numDer of slender 

 tracheae, and occupies the interspaces of the 

 various abdominal organs. Each lobe con- 

 sists of an outer structureless membrane, 

 enclosing the fatty matter imbedded in an 

 amorphous or granular substance. It ap- 

 pears to form a reservoir of nourishment for 

 the insect during the pupa-state. 



In most insects are found several slender 

 and elongated, mostly simple, tubular glands, 

 opening by simple or united ducts into that 

 end of the true stomach corresponding to 

 the pvlorus (PI. 35. fig. 2 e). Their free ends 

 are either caecal or unite with each other. 

 They are often very long, and much con- 

 voluted around the intestines, sometimes 

 presenting a varicose appearance, and di- 

 lated near their termination. These are the 

 Malpighian vessels j and they probably per- 

 form the f unction of a kidney, uric acid 

 having been found in them. They are 

 usually yellowish or brownish, and consist 

 of a homogeneous outer coat lined with 

 epithelial cells. Some authors, however, 

 consider that the renal organ is represented 

 by one or more long vessels convoluted 

 upon the colon and opening close to the 

 anus ; and we have found in the cater- 

 pillar of the fox moth, Lasiocampa rtibi, 

 numerous long convoluted tubes, of a 

 milk-white colour, filled with octahedra 

 and prisms of oxalate of lime ; these termi- 

 nated in the rectum close to the anus by 

 verv slender ducts, whilst at the upper ends, 

 which reached to about the anterior third 

 of the body, they were coiled upon them- 

 selves, or united with each other. 



Other glandular or secreting organs also 

 occur in insects. Thus organs correspond- 

 ing to the cutaneous glands of theVertebrata 

 are often met with as rounded glandular 

 cysts diffused beneath the integument, and 

 called glandulae odoriferse ; they open at the 

 junction of the segments of the body, or at 

 the joints of the legs, by very short ducts, 

 and pour out a strongly-smelling secretion. 

 Spaulding describes in the mentum of the 

 bee, a spiral duct connected with certain 

 glands situated in the thorax. In other 

 insects, similar organs are concealed at the 

 posterior end of the body, and pour out 

 their secretion near the anus. Among the 

 Hymenoptera, the females are often fur- 



nished with a glandular apparatus which 

 secretes the poison of the STING. 



Spinning organs. A large number of those 

 insects which undergo perfect metamorpho- 

 sis are furnished in the larval state with 

 spinning organs, with the secretion of which 

 many larvae, before entering the pupa-state, 

 weave a cocoon or enclose a cavity in which 

 to pass their period of rest, while others use 

 this secretion for agglutinating foreign 

 bodies to serve the same purpose. The 

 glands secreting the silk consist of two long, 

 tubular caeca (PI. 34. fig. 16), which in a 

 more or less coiled state occupy the sides of 

 the body, and terminate anteriorly in two 

 narrow excretory ducts, dilated to form a 

 reservoir, and the common orifice of which 

 opens outside the mouth on a short tubercle 

 beneath the labiuni. The caterpillar is able to 

 compress the silken threads by the contrac- 

 tion of an angle formed by the two capillary 

 tubes at their point of union, and is thus 

 enabled to suspend itself by the threads. 

 The material of the silk is always colourless, 

 and derives the colour which it presents in 

 certain instances from a varnish secreted in 

 the reservoirs, and issuing along with the 

 former. 



The heaj-t in insects exists as a long con- 

 tractile dorsal vessel, constricted atintervals. 

 This terminates posteriorly in a blind end, 

 and is narrower in front. The posterior 

 portion performs the functions of a heart, 

 whilst the anterior represents an aorta, and 

 conveys the blood from the heart to the 

 body. From the mouth of the aorta the 

 blood passes without any vascular walls, in 

 regular currents taking all directions, and 

 running into the antennae, the extremities, 

 the wings, and other appendages, returning 

 as a venous current. The blood finally 

 forms two principal lateral currents di- 

 rected towards the end of the abdomen, 

 and, accumulating in the neighbourhood of 

 the heart, is broug-ht by its diastole through 

 the lateral valvular tissues existing in it, 

 whence it is again driven through the aorta 

 as before. The walls of the dorsal vessel 

 consist of longitudinal and transverse fibres, 

 surrounded externally by a very delicate 

 peritoneal layer. The cavity of the heart is 

 lined by another delicate membrane, which 

 in the constricted parts forms internal val- 

 vular projections, whereby the dorsal vessel 

 is divided into as many chambers as there 

 are constrictions. Each of these cardiac 

 chambers is furnished at its front end, right 

 and left, with a fissure which can be closed 



