HARDWICKE'S SCIENCE-GOSSIP. 



265 



NOTES ON THE FRONTAL SAC OF THE MUSCIMl. 



OMETIMES ques- 

 tions have been put 

 to me on the sense- 

 organs of insects 

 to which I could 

 not give, neither 

 could I find by 

 reference, a satis- 

 factory answer ; 

 this was rather 

 humiliating, and 

 led mc to try to 

 unravel some of 

 the mysteries of 

 the subject myself. 

 The pursuit is of 

 great interest, and 

 very fascinating, 

 wish that I 

 impart to 

 the great 



and I 

 could 

 others 

 pleasure that I have felt in the work. 



In the older flies, which are often more numerous 

 than welcome in our homes, the frontal sac is not to 

 be seen on the exterior, but in the young insect that 

 has lately escaped from the pupa case it may fre- 

 quently be seen as a whitish bladder-like protuberance 

 on the front of the head. I have met with very 

 little literature on this organ. Walker merely speaks 

 of it as being alternately projected and withdrawn. 

 Mr. B. T. Lowne, in his "Anatomy and Physiology 

 of the Blow-fly," has given a detailed description of 

 the sac, he says — 



' ' When the perfect fly emerges from the pupa, a 

 large protuberance occupies the front of the head 

 immediately above the antenna;. It consists of a 

 membrane, by the dilatation of which the insect bursts 

 off the anterior extremity of the pupa case ; when 

 fully dilated it is larger than the head, which is as 

 yet retained within the pupa; but as soon as the 

 latter escapes, this protuberance rapidly collapses and 

 is withdrawn into it. Until the integuments are 

 thoroughly hardened it may be made to protrude 

 No. 288.— December 1888. 



from the head by gentle pressure, but is not after- 

 wards visible externally ; by pressing back the facial 

 plate, however, a fissure is opened between it and the 

 occipito-frontal plate and cheeks, by which the 

 cavity formed by the retraction of the membrane 

 communicates freely with the external air. The 

 membrane is continuous with the integument at the 

 margins of the fissure ; it assumes a bright yellow 

 colour in the mature insect, and is thrown into 

 numerous plications ; its connections are best seen 

 from the interior of the head. The extent of its 

 surface is very great, and is much increased by being 

 closely covered with papillae. It has probably three 

 distinct functions ; first, it serves to break off the 

 anterior extremity of the pupa case ; secondly, it is 

 undoubtedly connected with the humming of the 

 insect, the facial plate being thrown into rapid 

 vibrations during the emission of sound ; and lastly, 

 it seems to receive numerous nerve filaments from the 

 antennal nerves, and is probably an accessory organ 

 of smell. From its position I propose the term 

 frontal sac to designate it." He further remarks 

 that— 



" The evidence that induces me to include this 

 organ under those of special sense is, that its extensive 

 surface, which is thrown into numerous plications, is 

 still further increased by being covered with blunt 

 papilla. Its internal surface is covered with yellow 

 pigment, and with a thin layer of nerve cells 

 connected with filaments from the antennal nerves. 

 As already stated its cavity is in free communication 

 with the atmosphere, and the air contained must 

 be frequently changed by the vibrations of the 

 antennal plate. 



"With regard to the respective share which the 

 antennae, palpi, and frontal sac, probably have in the 

 olfactory sense, we have little to guide us : but it 

 appears to me likely, that the olfactory organs of the 

 antennae enable the insect to perceive odours, which 

 are quite inappreciable to the vertebrata. The palpi 

 are probably gustatory organs, whilst I think it very 

 probable that the frontal sac, the nerve supply of 

 which is far more limited, and the structure of which 



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