252 Transactions of the Society. 



XIII. — Notes on the PygicVa and Cerci of Insects. 

 By Henry Davis, F.B.M.S. 



QRead IZ'Ji November, 1878.) 



Some years ago most microscopists quoted the pygidium of a flea 

 as being one of the best of definition tests, and although doubtless 

 it is now well known as being so variable, that for comparative 

 trials (where objectives are not tested on the same specimen) it is 

 practically of little value; still its delicate beauty, the puzzle as 

 to its function, and the fact of its being generally considered as an 

 organ unique amongst insects, keep it to the present day as an 

 object of abiding interest, and one without which no cabinet would 

 be called complete. 



As one of its early admirers, I gave it, some years ago, con- 

 siderable attention, and was able not only to convince myself that 

 the angular, square-shouldered outline of the rays in the areola, 

 thus figured in the ' Micrographic Dictionary,' has no foundation in 

 fact, but that those areolae possess some outer structure which 

 seems hitherto to have escaped notice. It has afforded me a some- 

 what malicious pleasure in chaUeuging those of my friends who 

 used high modern powers, to discover this structure for themselves. 

 They invariably failed. A small-angled ^-inch objective, fully twenty 

 years old, first showed that, looking on an areola as representing a 

 carriage wheel, a line proceeds outwards from the tire between 

 each spoke, and these lines being bounded by a circle, give resem- 

 blance to a wheel within a wheel : the new wheel or circular band 

 is, when the object is unflattened by pressure, at right . angles to 

 the plane of the inner wheel; the first forming the sides, and 

 the latter the bottom of a Httle pit, from the centre of which 

 springs the well-known fine long hair. 



This is not brought forward for the purpose of glorifying old 

 objectives, or decrying the power, optical and manipulatory, of 

 certain microscopists, but rather to show the advantage of mounting 

 one's own preparations ; for the structure can only be well made out 

 when the object is placed in a position which a professional mounter 

 would endeavour to avoid and consider as wrong side out. 



Until 1870, when Mr. Peake discovered a pair of pygidia on 

 the Lace- wing fly [Ghrysopa jpeola), the Flea appears to have been 

 the only insect known to possess this appendage, and, after dihgent 

 inquiry, I cannot find that since that date any published addition 

 has been made to the number. But in December, 1870, it was my 

 fortune to notice two pygidia on a fine Locust (Locusta migratoria) 

 I had captured near Cadiz, and after finding these the road was 

 made to very many discoveries in other, mostly allied, insects. It 



