154 



KNOWLKDGi:. 



Al-Rll., 1912. 



may bo addcil and llifv can be left in this mixture for any 

 length of lime convenient. After a few days, they will lose 

 their brittleness. and c.in be opened out with needles. They 

 will be found to be filled with long narrow sausage-shaped 

 bodies, with very thin transparent walls, sometimes ditlicnit 

 to see. These are tisci and are interspersed — packed in as it 

 were — with fine threads — {he panipliyscs (Figure 176, a). The 

 asci each contain eight rather long, generally slightly curved 

 spores, pointed at each end. and divided by from five to 



eleven cross walls (Figure 17fi, b.) They arc arranged in a 

 double row with the ends overlapping, frequently in a spiral 

 manner, see Figure 176, a. It may happen that the fiasU-shaped 

 body contains only minute simple spores, free in the interior 

 without any asci. in which case the specimen is an incom- 

 pletely developed form of the same organism and has received 

 the name Apospaeria acuta — vide Dr. Cooke's " One thou- 

 sand objects for the microscope " No. 336. .iMmost certainly 

 another form exists also, and is probably included in the 

 genus Macrosporititu, as a species closely related to the one 

 we are considering (Sphaeria herbariiiii) is known in one 

 stage as Macrosporiiiin sarciniila, vide " KNowLEtiGE " 

 for October, 1911, page 406. The asci and spores mount 

 nicely in glycerine jelly, or they may be stained and mounted 

 in glycerine. The figure was drawn from a specimen treated 

 in this w^ay ; the parapliyses (a) XISO and the separate 



spores (b) X310. , „ 



'^ Jas. Burton. 



THE PYGIDIUM OF A FLEA, as mounted by Topping, 

 was formerly sold as a test object, but although in consequence 

 of the improvements in objectives it is no longer regarded as 

 such, it is still an object of great interest to the microscopist 

 on account of its intricate structure, and it has also a claim for 

 consideration inasmuch as it represents a class of organs 

 which are comparatively rare in the insect world. .A. 

 pygidium, properly so called, is the terminal segment of an 

 insect's abdomen, whether or not it bears the peculiar 

 structure usually known by that name. " The pygidium of a 

 flea " as commonly understood, is a paired organ, the two 

 halves of which are in close proximity, enclosed in the same 

 band of chitine and separated only by a narrow ridge of short 

 spiny hairs ; the length of the ridge in the specimen before 

 me is one-seven hundred and fiftieth of an inch, whilst the 

 breadth of the entire organ, including the ridge is one four- 

 hundred and eightieth of an inch. The anterior margin of 

 the band is smooth in outline, but the opposite side terminates 

 in two processes each of which carries a long stifl' spine and is 

 surrounded by numerous hairs of shorter growth. The 

 general surface of each lobe of the pygidium of a flea is 

 studded with minute papillae, short and bulbous at the base, 

 but tipped with a sharp apical spine, and amongst these are 

 placed a number of curious circular depressions, or areolae, 

 the two-thousandth of an inch in diameter, bordered by a ring 



of chitine from the inner margin of which seven to ten 

 triMicated wedge-shaped plates, about one-<|uarter the diameter 

 of the disc, r.adiate towards the centre, somewhat like the 

 spaces between the spokes of a cart wheel. From the bottom 

 of each depression a long delicate filamentous hair arises from 

 a conical base and passes through the central opening of the 

 areola. The number of areolae in the pygidium varies 

 according to the species of flea — in that of a cat it is twenty- 

 eight, but in lh.it of an abnormally large specimen of doubtful 

 origin in the cabinet of the Ouekett Microscopical Club there 

 are no less than sixty-four, together with a pair of remarkable 

 appendages extending from either side of the anterior portion 

 of the pygidium, which have not been noticed elsewhere. 

 As regards the function of this organ there has been 

 much speculation. All observers agree that it is a sense 

 organ, but of what sense has been a matter of dispute, 

 though the balance of evidence is certainly in favour 

 of its being an organ of hearing. Those who have watched 

 a flea when feeding will have noticed that whilst the 

 head is so much depressed as to preclude observation of its 

 surroundings by the eyes, the abdomen is so elevated as to 

 make the pygidium the highest portion of the body, in which 

 position it would undoubtedly be best situated for perceiving 

 the sound of approaching danger. The Lace-Wing fly, 

 Clirysopa perla, has a similar organ on either side of the last 

 segment of the body, the use of w^hich is also problematical ; 

 the following observation, however, tends to support the idea 

 that the function of the pygidium is auditory. 



Some years ago a friend in Natal informed me that he had 

 frequently noticed that when a Cicada was singing, it was 

 attended by a number of Lace-Wing flies which were 

 apparently attracted by the music, and from their movements 

 appeared greatly to appreciate it. Many futile attempts were 

 made to capture some of them, but as soon as my friend's 

 approach was perceived the Cicada ceased its song and the 

 audience took flight. .-\t length, having one day noted their 

 exact position on a tree trunk, he approached stealthily from 

 behind, and by suddenly clapping his hands upon the place, 

 succeeded in securing ten of the flies in question, which were 

 subsequently identified as Xotoclirysa ^igantca. The 

 method of capture did not conduce to the specimens being in 

 very good condition, and being very dry when they reached 

 England it was not possible to dissect them with hope of 

 success, but enough was seen to show that each of these flies 

 had a large and prominent 

 pygidium on each side of 

 the last segment of the 

 abdomen, each of which 

 bore not less than forty 

 areolae w-ith a long sensitive 

 filament upstanding from 

 the centre, although the 

 radiating plates appeared to 

 be absent. i; T I 



THE FINE ADJUST- 

 MENT. — Provision for the 

 focussing movements of a 

 microscope is made by 

 means of slides, consisting 

 of fixed and moveable parts 

 accurately fitted together, 

 one of them carrying the 

 tube. The two parts are 

 either dove-tailed one into 

 the other, or made up of a 

 sleeve fitted over a fixed 

 prismatic bar, usually of 

 triangular section. .A micro- 

 meter screw is used to 

 convey movement directly 

 to the slide bearing the tube, 

 or indirectly through a lover 

 or some other mechanical 

 arrangement for reducing 



FiGUKli 177. 

 Prismatic Bar Fine Adjustment, 



