174 



HABDWICKE'S SCIENCE- GO S SIP. 



associated with the approaching assumption of the 

 pupa state; but I have not as yet heen able to 

 satisfy myself that the larvae iu which I observed it 

 were in a more advanced condition than others in 

 which I saw it not. I hope perhaps to have some 

 future opportunity of satisfying 'myself on this point. 



The organs of respiration consist, as in other 

 insects, of ringed tubes called trachea;, opening on 

 the surface of the body, and ramifying over the 

 muscles and viscera, &c, their finer terminations 

 being distributed to every portion cf the organism. 

 The spiracles, which are the openings of these 

 tubes, are in general placed along the thoracic and 

 abdominal segments on each side ; but in the larvae 

 of the Diptei-a this arrangement is, I believe, not 

 followed ; and in the present case two spiracles only 

 exist, which are found in what I take to be the last 

 segment of the body. From these spiracles two 

 main trunks arise, which traverse the whole length 

 of the creature on its upper or dorsal surface. In 

 each of the abdominal segments two branches are 

 given off from each main trunk ; the smaller or 

 dorsal branch on each side joins with its fellow of 

 the opposite side, in the central line of the back, 

 between the dorsal vessel and the skin, from whence 

 a few smaller ramifications are given off. The 

 larger or ventral branches in each segment, after 

 seeding each an offset to the viscera, subdivide, One 

 oranch passing forwards and the other backwards, 

 to join those in the contiguous segments; thus 

 forming a sort of subsidiary trunk, from which two 

 branches in each segment are distributed to the 

 muscles, the larger of which following the course of 

 the nerves, ends by ramifying over the ganglia and 

 ventral cord, between the nervous substance aud its 

 outer coat. 



The distribution of the tracheae in the thoracic 

 segments is exceptional. The secondary trunk 

 ceases with the eighth pair of branches (reckoning 

 these backwards from the spiracle), beyond which 

 we find only one more offset, No. 9, from the main 

 trunk, previous to its final break-up into a number 

 of branches, two of which pass into the head, and 

 others, together with those from the ninth offset 

 just mentioned, proceed to the thoracic ganglia, the 

 imaginal discs, and the muscles of this portion of 

 the body. One of these terminal branches is re- 

 markable for the absence of the usual rings in its 

 wall; it ends in the superior prothoracic imaginal 

 disc, which corresponds, I believe, in the imago, 

 with the anterior thoracic spiracle. The trachea; 

 appear to consist of an internal and external wall, 

 iu the former of which the ringed structure is de- 

 veloped. Between the two is a space, more or less 

 considerable, varying I believe according to the 

 condition of the insect with reference to a moult. 

 This space appears to be occupied with formative 

 matter in a viscid condition, and so interspersed 

 with vacuoles, or clear oval spaces, as to give the 



impression that it is undergoing differentiation into 

 a cellular structure. I have frequently seen the 

 remains of a cast-off tracheal tube inside the new 

 one ; at other times this duplication, from the still 

 perfect condition of the inner tube, induces me to 

 think that it has not yet been detached, but that the 

 outer one is in course of formation around it, its 

 constituents being probably derived from the viscid 

 protoplasmic matter by which it is surrounded. The 

 five terminations of the tracheae are destitute of the 

 rings, and at certain points, as at the commencement 

 aud terminations of the main trunks, the ringed 

 structure is replaced by a sort of felt, of exceed- 

 ingly minute and closely-matted hairs. This is 

 especially the case for some little distance from the 

 spiracles, where also a new feature becomes strik- 

 ingly noticeable ; the walls of the tube at this 

 portion being pierced with a great number of small 

 pores or openings, from which issue very short 

 tubes, which immediately split up into minute 

 tufts of very fine tubules, in which, as usual, the 

 ringed structure is absent. These tubules occupy a 

 considerable space, and under a low power look like 

 a mass of white cotton thread : some of them pass 

 into the appendages by which the spiracles are 

 surrounded. I am inclined to think that this may 

 be a special provision for the more effectual aeration 

 of the circulating fluid, previous to its passage 

 through the dorsal vessel to the important organs of 

 the head, especially as the absence of anterior spi- 

 racles might seem to countenance the supposition 

 that the function of respiration in these parts might 

 otherwise be ill provided for. 



The spiracles are fair samples of that wonderful 

 perfection and finish which so frequently excites 

 the astonishment and delight of the microscopic 

 inquirer in the field of insect anatomy. They con- 

 sist of a central horny plate, from which radiate a 

 great number of branching processes, connecting it 

 with the circumference, like the spokes of a wheel. 

 These processes are only clearly visible from the 

 inside, as externally they are beset with rows of 

 very minute knobbed and slightly forked hairs 

 pointing in a sloping direction towards the centre of 

 the spiracle in a plane perpendicular to its surface. 

 The whole is surrounded by a beautiful ornamental 

 edging, the great number and regular setting of the 

 knobbed hairs rendering it a very attractive object 

 for the cabinet, when properly cleaned and mounted 

 in balsam for that purpose. 



No closed system of arteries and veins exists in 

 insects, as in the higher division of the animal 

 kingdom, the circulation of their nutritive fluids, as 

 is well known, being effected by a single pulsating 

 vessel, which runs along the centre of the back 

 immediately beneath the integument. This vessel 

 is divided into a number of chambers, and Bur- 

 meister* describes these as furnished with valves 



* Shuckhard's Translation, 1836, pp. 154 and 155. 



