CHAPTER OF VARIETIES. 181 



slender stalk of a fir tree during, March, 1832. I carefully placed them 

 in a breeding box, and in a few days they were hatched, and a number 

 of small flies made their appearance, having black bodies, four trans- 

 parent wings, the upper pair having a pretty tinge of changeable hues, 

 and a small square black spot in the middle of the exterior margin, 

 the hind pair of legs disproportionately long, and of a rather dark red 

 colour, and the antennae filiform, and bending upwards like an in- 

 verted arch. They were submitted to a gentleman distinguished in 

 this department of natural history,' and they were returned to me 

 labelled, (Micorogaster alvearius, FABRICIUS,) which is a species of 

 parasite. When found, the whole mass of eggs were covered over 

 by a very thin film of silk, of a light pink colour, and the apertures 

 from which the flies made their exit were not very apparent until they 

 were near upon opening, which, when they did, the round lid of each 

 cell turned back as though by a hinge, and the cells were divided 

 from one another by a thin delicate partition. I think it not impro- 

 bable that the eggs were deposited by a spider, as I have often found 

 them in somewhat similar situations, and, like the present, protected 

 by a thin web ; but as that is only conjectural, I^should feel obliged if 

 you could solve the mystery. SOLIVARIUS. 



EXPERIMENTS ON THE AIR CAVITIES AND THE COLOURS OF LEAVES. 

 In a late number of the Annales des Sciences Naturelles ap- 

 peared a paper by M. Dutrochet, stating some experiments in which, 

 for the purpose of showing the air cavities in leaves, he put them, sub- 

 mersed in water, under the exhausted receiver of an air pump. The 

 pale underside of the leaf thus treated assumes the deep green colour 

 of the upper surface. The air leaving its cavities allows the water to 

 enter and fill them. On repeating these experiments I observed, that 

 as the water evaporated and the underside of leaf resumed its colour, 

 it did so not in all the parts of it at the same time, but the air cavities 

 afforded different degrees of facility to the evaporation j and of con- 

 tiguous cavities some of them remained for some time full of water 

 and of a deep green colour, while others were dried up and filled with 

 air, having" regained their paler colour. By this means, the form, 

 size, and arrangement of the air cavities were easily distinguishable. 

 The enquiry naturally suggested itself whether, if, instead of water, 

 a coloured fluid were used, the cells might not be permanently tinged 

 so as to become perceptible to the eye. Red and black inks were tried, 

 but without effect, and from' these and other trials it appeared highly 



