INSECM. 



981 



Now the opinion to which we have been led by 

 our examinations of dried specimens, as we have 

 not been so fortunate as to obtain living ones, 

 since the commencement of these observations, 

 is, that every nervure contains a distinct tra- 

 cheai or air-bearing vessel, and that the free 

 transparent space by which it is surrounded, 

 and which is not discoverable by the naked 

 eye, but only by the microscope, constitutes 

 alone the proper circulatory passage; that the 

 tracheae, which are obvious to the naked eye, 

 do not simply lie loosely in these spaces, but 

 that the spaces lie chiefly at their sides and 

 under-surface. These opinions have been de- 

 rived from examinations of transverse sections 

 of the wings of the Chrysopa, at parts in which, 

 on a previous examination of the surface of 

 the wing, we have seen both trachea? and the 

 spaces around them. On cutting the wing 

 across at these parts, and then examining the 

 edges, we have invariably found the tracheae 

 hollow, unyielding tubes, while the free spaces 

 on each side, which appear as if bounded by 

 the upper and under membrane of the wing, 

 have appeared collapsed, and almost or com- 

 pletely closed, so as scarcely to exhibit any 

 appearance of hollow spaces. On examining 

 the wing of a dried specimen of one of the 

 Lepidoptera, Gonepteryx Rhamni, by trans- 

 verse sections, we have in every instance found 

 the nervures formed of hollow unyielding tubes, 

 with all the characters of true tracheal vessels, 

 but have not been able to detect the proper 

 circulatory spaces at the sides of these nervures, 

 most probably owing to their dried and col- 

 lapsed state. From these facts we are led to 

 express an opinion which has been long enter- 

 tained by us, that the course of the blood, 

 whether simply along intercellular spaces, or 

 bounded by distinct vessels, is almost inva- 

 riably in immediate connexion with the course 

 of the tracheae. This opinion is founded upon 

 the circumstance that nearly all the observations 

 that have hitherto been made have shown that the 

 currents of blood in the body of an insect are 

 often in the vicinity of the great tracheal vessels, 

 both in their longitudinal and transverse direc- 

 tion across the segments,and it is further strength- 

 ened by Mr. Bowerbank's observations on the 

 course of the blood in the wings. During his 

 observations Mr. Bowerbank observes that he 

 " has used every endeavour to discover, if pos- 

 sible, whether the blood has proper vessels, or 

 only occupied the internal cavities of the ca- 

 nals ; and that he is convinced that the latter is 

 the case, as he could frequently perceive the 

 particles not only surrounding all parts of the 

 tracheae, and occupying the whole of the in- 

 ternal diameter of the canals, but it frequently 

 happens that globules experienced a momen- 

 tary stoppage in their progress, occasioned by 

 their friction against the curved surface of the 

 tracheae, which sometimes gave them a rotatory 

 motion." He remarks also that the usual course 

 of the blood through the canals is in one conti- 

 nued stream, without pulsatory motion, ex- 

 cepting only when the insect under examination 

 is struggling to escape, when the continuity of 

 the stream is broken, and there are occasional 

 oscillations, n which he observed, iu one in- 



stance, in a vessel within a space ot about one- 

 fiftieth of an inch, twenty-one oscillations in a 

 minute ; and in another, in the same space, so 

 many as eighty-four. He observed also in in- 

 sects that had been captured and in confine- 

 ment for several days, that the motions of the 

 fluid became exceedingly languid and almost 

 entirely ceased. Thtse observations are exceed- 

 ingly interesting in reference to the general 

 velocity of the circulation, and the means by 

 which it is carried on in the wings. The entire 

 absence of pulsations is remarkable, as it com- 

 pletely identifies these vessels as veins, since it 

 is well known that the circulation is carried on 

 through the body by means of regular pulsa- 

 tions of the dorsal vessel. The number of these 

 latter pulsations varies greatly in different insects. 

 Thus Herold found from thirty to forty in a minute 

 in a full-grown caterpillar, and from forty-six to 

 forty-eight in a much younger one; Suckow ob- 

 served but thirty, in the same space of time, in a 

 full-grown caterpillar of Gastropacha pini, and 

 eighteen only in its pupa state. But in one in- 

 stance, when the insect was in a state of the most 

 violent excitement, we have counted one hundred 

 and forty-two in a female of Anthopkora re- 

 tusa. In a number of these insects captured 

 just after they had left their hybernacula in the 

 month of April, and confined for some time in 

 a breeding-cage, we have found that the num- 

 ber of pulsations varies, as might, a priori, be 

 supposed, according to their state of excite- 

 ment. Thus on exposing the dorsal vessel in 

 the morning, before the insects had been excited, 

 we found the number of pulsations was about 

 eighty per minute ; at ten o'clock, when they 

 had become active, the number of pulsations 

 ranged from one hundred to one hundred and 

 ten ; but at three o'clock in the afternoon, when 

 the insects were quite lively, and had been 

 exposed to the sun for an hour or two, the 

 number of pulsations amounted to one hundred 

 and forty; while on another occasion, on a cold 

 dull morning, when the bees were languid, the 

 number of pulsations did not exceed seventy- 

 five in any specimen that was examined. It 

 has usually been supposed, since the discovery 

 of a circulation in insects, that the pulsations 

 are more frequent in the larva than the perfect 

 state ; but this certainly is not the case, if the 

 mean number of observations in the two states 

 be compared. Thus in a series of observations 

 made by us on the Sphinx ligustri,* from the 

 fourth day after the larva had left the egg until 

 the perfect insect was developed, it was found 

 that before the larva cast it&Jirst skin the mean 

 number of pulsations, in a state of moderate 

 activity and quietude, was about eighty-two or 

 three per minute ; before casting its second skin 

 eighty-nine ; while before casting its third it 

 had sunk down to sixty-three ; and before its. 

 fourth to forty-five, while previously to leaving 

 its fourth, and before it had ceased to feed, pre- 

 paratory to entering the pupa state, it was not 

 more than thirty-nine. Thus the number gra- 

 dually decreases during the growing larva state, 

 but the force of the circulation is very much 

 augmented. Now when the insect is in a slate 



* Phil, Tians. p. 2, 1837. 



