156 



SCIENCE- GOSSIP. 



adaptability of the abdomen for rhythmical ex- 

 pansion and contraction leaves no doubt that the 

 air is obtained through these spiracles. But that 

 the tracheae branch and ' re-branch until they 

 end in mere blind twigs, and that the air is 

 constantly being changed in them, is, to say 

 the least, unsatisfactory. By mounting fresh 

 insect muscle in strong glycerine, I have suc- 

 ceeded for a time in retaining air in the smallest 

 tracheal vessels, and believe that I have traced a 

 connection through them to and from the larger 

 tracheae. There is so much difficulty in following 

 them that 1 wish it confirmed by more able 

 observers before it can be established as a fact. 

 On the other hand, the thoracic spiracles are large, 

 and in many insects op 3n outwards, so that there 

 is little or no protection against the entrance of 

 dust particles ; whilst the various parts of the 

 thorax are so firmly soldered together as to make it 

 almost rigid. These spiracles appear purposely 

 adapted for carrying off the expired air and vapours 

 from the body. There is yet another purpose that 

 they may fulfil ; namely, the production of sounds. 

 The well-known buzzing of the fly can be varied 

 in tone, or may cease altogether, as circumstances 

 may require. Is it not conceivable that by varying 

 the size of the opening of the valves of these 

 spiracles, and thereby increasing or diminishing the 

 pressure in the large air-tubes connected with them, 

 such variations may be accounted for ? When the 

 wings and halteres of the fly are removed, the 

 sounds produced are as loud as before removal, and 

 the muscular exertion is always great when sounds 

 are produced. — William Jenklnson. 



Remarks on the Foregoixc4 Notes. 



With regard to the difficulty respecting the hairs 

 on the pulvilli of flies, is it to be expected that the 

 hairs should be hollow and in the nature of ducts 

 for the viscid fluid secreted by the glands 1 Do 

 these hairs not act rather as a simple mechanical 

 method for enabling the insect instantaneously to 

 ■detach its foot-hold from the object upon which it 

 is resting ? And supposing the pulvillus to be hair- 

 less, and the secreting surface to be brought into 

 close connection with that object, would there not 

 be great difficulty in the creature at once liberating 

 itself? The function of the frontal sac is a very 

 interesting subject, and I am inclined to agree with 

 Mr. Jenkinson that it acts largely as a compen- 

 sating arrangement in the adjustment of the 

 pressure upon the organs of the head, in which the 

 protrusion of the proboscis and its retraction must 

 otherwise occasion considerable variation ; but, at 

 the same time, it doubtless fulfils other purposes. 

 1 have not yet seen Prof. Lowne's second edition 

 ■of his work on the blow-fly, in which, I believe, 

 he has in several ways considerably modified his 

 views on certain points enumerated in the first 

 ■edition. I do not know therefore whether his 

 views upon the frontal sac are in any way altered, 

 but I think he ascribes several functions to it. 

 The first of these was that this sac effected the 

 purpose of a lever, to enable the maturing imago 

 to escape from the pupa-case by forcing off the 

 ii] >per end and so permitting the insect to escape. 

 At this time the frontal sac, which forms a cavity 

 in the head of the mature fly, is everted and forms 

 a protuberance in front of the head, which, how- 

 ever, immediately after emergence collapses and is 

 withdrawn into the head, from which, by a slight 

 pressure, it can be again made, for a short time 



only, to protrude. The second function is in con- 

 nection with the humming of the insect. As 

 Professor Lowne says, the facial plate is caused to 

 vibrate rapidly during the emission of sound, 

 which, in the apparent absence of sufficient 

 muscular power, is difficult to conceive. The last 

 is — that it is an olfactory organ, though with a 

 limited nerve supply, by means of which the insect 

 is enabled to appreciate powerful odours. It seems 

 to me it will be difficult to establish Mr. Jenkin- 

 son's last suggestion as to the function of the 

 laterally opening tracheae in the interior of the 

 body by means of capillary vessels, or something 

 analogous. Under any circumstances the advant- 

 age of such connection would appear doubtful, 

 owing to the extreme minuteness of such capillaries 

 did they exist. Any exchange of air in the tracheae 

 which is effected by the respiratory movements of 

 the abdomen could hardly proceed more rapidly 

 one way than the other. — E. Bostoclt. 



I quite agree with Mr. Bostock that the hairs 

 beneath the pulvilli may act mechanically in help- 

 ing to release the foot. My difficulty is simply 

 that if the glands in the pulvillus secrete a viscid 

 fluid, how is that fluid conveyed to the under- 

 surface of the pulvillus 1 When I approached the 

 subject I expected to find a lumen in each hair, 

 or. failing that, minute ducts passing through the 

 external wall ; but I can find neither. The frontal 

 sac is continuous with, and is simply an invagina- 

 tion of, the epidermis. It is a flexible bag floating 

 iii and surrounded with fluid, and in my opinion 

 is unsuitable for producing quick vibrations, but 

 would rather tend to subdue them. Hence we 

 must look elsewhere for the "humming" organ. 

 It is equally unsuitable for an olfactory organ, 

 because its only nerve supply could be obtained by 

 its outer edges ; and these would be very limited, 

 even if they exist at all, which is extremely doubt- 

 ful. The experiment of placing the thorax of a 

 cricket under water, whilst the abdomen had free 

 access to the air, showed bubbles of air emitted 

 from the thoracic spiracles without the insect being 

 in the least exhausted. When the abdomen was 

 immersed no bubbles were emitted, and the insect 

 suffered from exhaustion. Such experiments, 

 coupled with the construction of the thoracic 

 spiracles, suggest that they are outlets for the 

 expired air. — Win. JenJtinson. 



[The exact method by which the breathing of 

 insects is carried on is still unknown, and there is 

 room therefore for much investigation. Eisig 

 suggested that the formation of chitin is the result 

 of excretion, thus relieving the system of carbon 

 dioxide to a certain extent. The theories of Lowne 

 and others as to inspiration and expiration are well 

 known, if not widely accepted. It has also been 

 suggested that transpiration is carried on through 

 the more delicate parts of the integument Into 

 other points raised by Mr. Jenkinson we need not 

 enter ; but we regret that we have not been able 

 to illustrate these notes more fully. Mr. Jenkinson 

 very courteously sent us such of the original slides 

 as lie still had by him, but some had been destroyed: 

 The photographing of the sections was a matter of 

 no little difficulty owing to their want of contrast; 

 but time did not unfortunately permit of our 

 making the elaborate drawings of them which 

 would have been necessary in order to illustrate 

 the foregoing notes adequately, whilst the sections 

 themselves are, as will be observed, somewhat im- 

 perfect for this purpose. — Ed. Microscopy, S.-G.] 



