1S94.] 61 



and engine) is a machine for the evolution of force from fuel — the muscles being 

 represented by piston rods (motors), the fat or fuel food by coal (motive power). 

 Perhaps a comparison of migratory birds with their insect analogies would have been 

 more appropriate, for in the latter case the feathered migrants stock themselves well 

 with fuel food before undertaking their arduous flights (see Eeports of the British 

 Association Committee, on the migration of birds) ; and the males beat the females 

 in the race (Harting : " Our Summer Migrants "). 



Another point upon which I touched is, that after pairing and oviposition, one 

 of the chief purposes for which energy was stored has been fulfilled, and the vigour 

 of the insects begins to wane. This case bears a resemblance to that of fishes at the 

 time immediately preceding the spawning season, when they are in plump condition ; 

 the milts of the males being then so charged with fat, that when thrown upon a 

 fire, they will flare up and burn for a considerable time. After breeding, fish become 

 flaccid and out of condition ; with moths it is the same, but having scarcely any 

 recuperative powers, they soon afterwards die. 



On other points Dr. Freer and I agree in the main ; we both concur in con- 

 sidering that internal feeders, which, as I observed, " live more or less in the dark, 

 secluded from air, and restricted as to movements," are among the most liable to go 

 greasy, i. e., to stock the largest quantity of fuel food ; but these are comparatively 

 only a small proportion of the bulk of "greasers," a large number of which were 

 disposed of in my suggestion that they stored fat for purposes of hibernation in the 

 larval or perfect state, just as in the case of hibernating vertebrates. How does Dr. 

 Freer account for the tendency to grease in many butterflies, all the Sphinges, most 

 of the Bombyces, besides a large number in each of the other groups of moths, 

 which do not spend their larval existence as internal feeders ? 



Then, of course, we agree that the fat gets out of the cells, but is it always by 

 putrefaction ? No doubt this takes place in a damp atmosphere, but, if I may be 

 allowed to say so, there is another way, of still more common occurrence, namely, 

 dessication, by which this result may be efEected, and this would happen when 

 the cabinet was kept, and the setting operations conducted, in a comfortably warm 

 dry room. Here the cell walls would contract, the contained oleaginous matter 

 would expand, and, as a consequence, the cells would be ruptured. 



I quite agree that dryness is the greatest prophylactic against the outward 

 appearance of grease, but it must be accompanied by coolness : — " on a hob " or " in 

 a cool oven " would be " dryness," but such a position would soon " start the grease." 

 — H. GuAED Knaggs, Camden Eoad, London, N.W. : February 3rd, 1894. 



Grease in old specimens of Lepidoptera. — Many of the insects of Mr. Gregson's 

 collection, which were in good order when they came to me five years ago, are now 

 showing signs of grease, and must be treated by one of the modern cures. Some of 

 these must be nearly ///y years old ! Why they never showed any tendency before, 

 and why it now happens, must probably arise from diiference of atmosphere, either 

 between Liverpool and Dover, or the heat of the room in which they have been 

 kept. Mr. Gregson I know is a firm believer in a circulation of air around cabinets, 

 and probably to this the long deferred evil may in great measure be attributed. — • 

 Sydney Webb, Maidstone House, Dover : February IZth, 1894. 



