266 



HARDWICKE'S SCIENCE-GOSSIP. 



is far coarser, is especially adapted for the appreciation 

 of the more powerful odours." 



From these quotations it is quite evident that the 

 knowledge of the functions of the frontal sac is very 

 imperfect, but as Lowne has made two affirmatives, 

 and one inference, I have tried to confirm him. 



As the sac in its earliest stage is external to the 

 head, contains undeveloped cells, which for want 

 of a better term I will call growth cells, and fluid, 

 it is well adapted to the forcing open of the pupa 

 case. 



With all due deference to Mr. Lowne's professional 

 knowledge, I think that he is somewhat wide of the 

 mark in stating that the frontal sac is connected with 

 humming of the insect. Besides the brain, nerves, 

 muscle, air-sacs, etc., the head of the insect contains 

 a large quantity of fluid which will necessarily be in 

 contact with the inner surface of the sac, and thus 

 help to counteract any vibrations of its walls. Again, 

 if it remained perfectly diy, the loose plications into 

 which it is formed would yield if the least tension was 

 applied, and thus be anything but conducive to the 

 production of sound. The absence of any muscular 

 attachment to its walls, or other means of compression 

 by which the contained air could be forcibly driven 

 through a narrowed outlet puts this means 'beyond a 

 doubt, even if it was not borne out by the fact, that 

 there is no provision for regulating the size of the 

 outlet, if it was capable of alterations in size, which 

 it is not. The facial plate, as well as the whole head, 

 doubtless vibrates ouring the production of sounds 

 by the insect, though it does not follow that the sound 

 must emanate from the head. Any one who has 

 watched a grasshopper when it gives its shrill call, 

 must have noticed the great muscular exertion 

 required to produce the sound, and probably it is so 

 with most insects that have this gift. From the few' 

 observations I have made, I think that we shall have 

 to give more attention to the thoracic spiracles before 

 the question of sounds produced by insects is cleared 

 up. 



The inference that the sac is an olfactory organ 

 seemed quite feasible, and as Mr. Lowne has given 

 his reasons for arriving at that conclusion, I have tried 

 to follow him. First, I sought for the nerve fibres 

 given off by the antennal nerve to the sac, then for the 

 layer of nerve cells covering the internal surface of its 

 thin walls, but have failed to find either. The yellow 

 pigment is represented in the young fly by the un- 

 developed mass of nucleated cells, which disappear as 

 the various tissues become perfected. The papilla? 

 appear to be simply thickened elevations of the walls 

 of the sac, without any perforation. Epidermal papilla; 

 are present in the base of the halters, but these have a 

 distinct lumen into which a nerve penetrates, but in 

 the papilla.- of the frontal sac, nothing of the kind is 

 to be seen. I have therefore reluctantly come to the 

 conclusion, that it is not sensor}', but serves some 

 purely mechanical purpose. 



Having reached this conclusion, its purpose must be 

 further sought for, therefore, looking at the whole 

 head, I found that the boundary on every side save 

 one, was composed of hard inflexible chitine, but 

 fiom the one side projected the flexible proboscis. 

 The house-fly and other insects of the same genus 

 have the power of projecting and withdrawing the 

 proboscis at will. I have before stated that the head 

 contains a large quantity of fluid, so does the 

 proboscis. There are also large air-sacs in the head, 

 but these are constructed on a plan to resist external 

 pressure, in some instances they are strengthened by 

 a lining of spiral fibre. Now for a moment, suppose 

 the proboscis to be projected to its full extent, what 

 would happen if it was withdrawn and pressed up 

 towards, and partially into the lower part of the head ? 

 The action would most certainly displace some of the 

 fluid contents both of the head and proboscis ; fluids 

 being inelastic, some provision must be made for the 

 displacement, or otherwise the act could not be 

 performed. Now the frontal sac, with its elastic folds 

 hanging so gracefully in the fluid, appears to be the 

 very best and simplest organ for the purpose. 

 During the act of projecting the proboscis the air 

 would rush in through the opening in the forehead 

 and inflate the sac, causing its outer walls to become 

 further separated and fill up the space vacated by the 

 fluid. When the proboscis is again retracted, a 

 reverse action of the frontal sac would take place, the 

 air would be expelled and the external walls of the 

 folds approach each other. Thus a uniform pressure 

 on the brain, nerves, and muscles would be main- 

 tained, whatever might be the position of the 

 proboscis. 



Now this is all very plausible, and things fit as they 

 should do in any well-arranged place, but will it bear 

 testing? With this object in view, I have cut into 

 sections, heads of flies of one and the same species, 

 some with the proboscis distended, others with it 

 retracted. In those with the proboscis distended, I 

 found the space occupied by the frontal sac with its 

 folds proportionately large, in those with the pro- 

 boscis retracted, the folds of the sac are compressed, 

 and the whole lower portion is elevated, and ap- 

 proaches the upper external wall ; in a word, it occupies 

 less space in the head than when the proboscis was dis- 

 tended. The uses of the papilla? are now apparent, 

 for as the external walls of the sac are brought closer 

 together the papillae interlock and retain a certain 

 portion of air, thus preventing the surfaces adhering 

 to each other, which might be the case if they were 

 perfectly smooth and damp. 



I have had to bring this subject before you much 

 sooner than I intended, therefore, have not had the 

 opportunity of confirming this view of the organ, so 

 thoroughly as I should wish, but should it, on further 

 investigation prove to be wrong, and a right one 

 promulgated, I shall be as ready to embrace it as any 

 one. In justice to Mr. Lowne I would remark, that 



