INSECTS. 



[ 360 ] 



INSECTS. 



many insects, caecal appendages arise from the 

 latter, and also contain cells which secrete a 

 biliary liquid. 



In some insects the small intestine is fur- 

 nished with glandular appendages in the 

 form of tubular caeca, probably representing 

 a pancreas. 



Intimately connected with the digestive 

 and assimilative process is a curious organ 

 called the fatty body. This attains its maxi- 

 mum of development towards the end of the 

 larval period of existence. It consists of a 

 number of fat-cells imbedded in a reticular 

 or lamellar tissue (PI. 28. fig. 28), composed 

 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 number of slender 

 tracheae, and occupies the interspaces of the 

 various abdominal organs. Each lobe con- 

 siats 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 pylorus (PI. 28. fig. 2 e). Their free ends 

 are either caecal, or unite with each other. 

 They are'often very long, and much convoluted 

 around the intestines, sometimes presenting a 

 varicose appearance, and dilated near their 

 termination . These are theMalpighi an vessels, 

 and they probably perform the function 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, how- 

 ever, consider that the renal organ is repre- 

 sented by one or more long vessels convo- 

 luted upon the colon, and opening close to 

 the anus. And we have found in the cater- 

 pillar of the fox-moth, Lasiocampa rubi, 

 numerous long convoluted tubes, of a 

 milk-white colour, filled with perfect octo- 

 hedra and prisms of oxalate of lime. 

 These tubular organs terminated in the 

 rectum close to the anus by very slender 

 ducts, whilst at the upper ends, which 

 reached to about the anterior third of the 

 body, they were coiled upon themselves, or 

 united with each other. 



Other glandular or secreting organs also 

 occur in insects. Thus, organs correspond- 



ing to the cutaneous glands of the Vertebrata 

 are often met with as rounded glandular 

 cysts diffused beneath the integument, and 

 called glandulae odoriferae ; 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. 

 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 furnished 

 with a glandular apparatus which secretes 

 the poison of the STINGS. 



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 (PL 27. 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 labium. The caterpillar is able to 

 compress the silken threads by the contraction 

 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 heart in insects exists as a long con- 

 tractile dorsal vessel, constricted at intervals. 

 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 abdominal appendages, 

 returning as a venous current. The blood 

 finally forms two principal lateral currents 

 directed towards the end of the abdomen, 

 and, accumulating in the neighbourhood of 

 the heart, is brought by its diastole through 

 the lateral valvular fissures existing in it; 

 whence it is again driven through the aorta 



