294 



SCIENCE- GOSSIP. 



THE PTILINUM OF DIPTERA. 



By W. WeschE. 



/^F the extraordinary and the apparentl)' ingenious 

 ^^ in Nature, there is no end. No limit can be 

 placed to her resources or to her inventiveness ; 

 almost every mechanical law has been adapted by 

 some plant or animal, every philosophical principle 

 applied. The orchid has used the catapult and the 

 sucker to fix pollen on the body and head of the 

 visiting insect ; the dandelion, the parachute to scatter 

 its seeds ; the bombardier beetle, the chemistry of 

 explosives to frighten its pursuing enemy ; the eel, 

 electricity for defensive and offensive purposes ; the 

 bees and the wasps, a nicety in the use of poisons 

 that our most learned toxicologists must vainly hope 

 to emulate. It is not generally known however, 

 that our familiar flies avail themselves of pneumatic 

 force to escape from their pupa cases : yet such is the 

 fact with a large number of the families of Diptera. 



Baron Osten Saken has arranged the order Diptera 

 into three sub-orders — the Pupipara or degraded 



Fig. I. l^fi-^A oi Scaiophagalutaria. 



forms, the Cyclorhapha or those having a circular 

 opening to the pupa case, and the Orthorhapha or 

 those having a T-shaped opening — which is further 

 subdivided into two sections, Brachycera, consisting 

 of flies having short three-jointed antennae, and 

 Nematocera, of those having many-jointed antennae. 

 Mr. Verrall, whose list of British Diptera is, I believe, 

 the only one in any degree complete, and whose 

 arrangement is more generally used by British col- 

 lectors, begins at the other end, but includes the 

 Pulicidae or fleas, which Professor Packard places in 

 a separate order, Siphonaptera. The Pulicidae head 

 the list of the Orthorhapha, and the degraded forms 

 are at the end of the Cyclorhapha. 



Many of the flies of the sub-order Cyclorhapha are 

 furnished with the ptilinum. It is a membrane on 

 the head, situated on that part immediately between 

 the occelli and the antennae. This is capable of 

 inflation until it takes a nearly spheroidal shape, and 

 projects, expanding to about half the size of the 

 head. By this means, without any struggle of the 

 limbs, the perfect insect pushes off the cover of the 

 circular aperture of the pupa case. There are, how- 

 ever, certain families that are without the ptilinum, 



though they are included in this sub-order ; those 

 flies who have nearly the whole head covered with 

 eyes, as Pipimcuhis and Syrphus. and the less- 

 known though well-marked families of Phoridae and 

 Ephydridae. On the other hand it is very distinct in 

 Mclophagiis oi'iims (sheeptick), and I find traces of 

 it in Braiila caeca (bee-louse). Both these parasites 

 are degraded forms, and belong to the sub-order 

 Pupipara. 



It is unfortunate that no trace of this apparatus is 

 visible on the insects as they usually appear w^hen 

 pinned in our cabinets. Preparation for the micro- 

 scope is necessary to show it, as otherwise it would 

 be of considerable service in classification. The 

 common dung fly, Scatophaga stercoraria, shows the 

 ptilinum admirably, and is easily obtained. 5. hitaria 

 does equally well, and the accompanying drawings 

 are made from that species. 



Fig. I shows a drawing of the ordinary appear- 



FlG. 2. 



Head (AS. lutaria, showing membrane forming 

 ■ ptilinum. 



ance of the head as seen from the side. The eyes, 

 bright red in life, become quite darji ; the general 

 colour is a shade of olive-green, and the facies, the 

 space between the eyes, is an orange-yellow. 



Fig. 2 shows the top of the head viewed from 

 above, with the occelli placed on the curious triangle 

 which in Scatophaga juts out on to the orange- 

 coloured facies, which is the membrane of which the 

 ptilinum is formed. 



Fig. 3 shows a head, prepared for the microscope 

 and mounted, under pressure. The pressure expands 

 the ptilinum, which is seen to project as if inflated. 

 In life, after use, it is wrinkled up into many folds, 

 and remains in that condition. Sometimes it fails to 

 expand when mounted, remaining in this state ; but 

 from the peculiar structure of its surface it is easily 

 detected by the microscope with a magnification of 

 250 diameters. 



Fig. 4 shows a portion of the ptilinum more 

 magnified. Its surface is seen to be covered with a 

 number of minutely pointed angular scales. These 

 on the expansion of the membrane, which is prob- 

 ably in some degree elastic, become erect. These 

 scales vary in appearance and size in different 



