212 



KNOWLEDGE 



[Skptbmber, 1903. 



to tbe delicate nature of the work carried on beneath its 

 modern dome. 



Thoui^h primarily designed to co-operate with the great 

 observatories in the work of the International astrographic 

 chart and catalogue, the photographic department of the 

 Vatican Observatory has by no means limited its activities 

 to mere routine work, but has secured several good solar 

 and lunar photographs in addition to its fine studies 

 of nebuliB and star clusters, while a speciality has been 

 made of cloud photography, the Vatican Ijeing well repre- 

 sented in tliis drjiartnu'ut at the Koyal Meteorological 

 Society's Exhibition, held in Loudon in 1890, for tbe 

 application of photography to meteorology.* 



Under Denza, who worked assiduously at his post as 

 director until his death in December, 1894, the Vatican 

 Observatory rose to renewed life and activity, and, con- 

 jointly with its sister establishment in Sicily, Catania, 

 entered with enthusiasm upon its share of that great 

 international undertaking which was to bequeath to future 

 times a photographic record of the entire heavens at the 

 close of the nineteenth century. It was, however, reserved 

 for Deuza's successor to the directorship. Father Rodriguez, 

 to whose kindness and courtesy I am indebted for many of 

 the above details, as well as for the accompanying photo- 

 graphs, to see this monumental work (which requires some 

 eleven thousand plates in order to cover the whole sky) 

 brought, after several minor interruptions, to its present 

 more or less completed condition. 



Conducted by M. I. Cross. 



THE COLLECTION, EXAMINATION, AND PRESER- 

 VATION OF MITES FOUND IN FRESH 

 WATER (IIi/drachiidcE). 



By Chas. D. Soar, f.r.m.s. 



Anyone with a love for natural history wishing for a hobby 

 for his spare time, would find the study of fresh-water mites 

 (ni/druchnidm) an extremely interesting one. For variety and 

 beauty in colour, and for differences in form and structure, they 

 are not to be surpassed by any other organisms found in fresh 

 water. Wherever there is a pond, ditch, or stream, the collector 

 is nearly sure of being rewarded for his search by finding one 

 or more species of these interesting creatures. They are easily 

 caught, and can be seen with the naked eye ; they are, however, 

 very seldom recognised without the aid of the microscope. 

 They can be kept alive for a considerable period at home, and 

 are easily preserved when killed. 



At present the life-history of these little creatures is so 

 imperfectly known that there is wide scope for an observant 

 naturalist. Although the Hfe-historiea of some species have 

 been fairly investigated, the number of such is very limited 

 compared with the species known, and the variety of species 

 which have been recorded in Great Britain are behind the 

 recorded collections of Germany and elsewhere. 



These creatures are caught in three distinct stages — the larval, 

 the nymph, and the imago. In the larval stage they are very 

 small, and only have six legs. When thej first emerge from the 

 egg they are free-swimming, but they soon become attached as 



* Keproductions of some of the Observatory's cloud photograplis 

 appeared in Knowledge for May, 1899. 



parasites to some other form of Pond-life. They will often be 

 found hanging like small red pear-shaped appendages on a great 

 number of acjuatic insects. The six legs they started hfe with 

 disappear after they have become firmly attached by their mouth 

 organs to their host, and they spend the remainder of this period 

 of their existence without any. 



This stage is succeeded by the nymph ; the little creatures 

 are then much larger and have eight legs. During this term of 

 their existence they are free-swimming, and can be caught in 

 the net in numbers, but it is impossible to distinguish the sexes. 



In the last stage, — the adult or imago, all the structure and 

 form are present, but many may be taken that are not fully 

 develo|)ed. In the majority of species, the male can be 

 distinguished from the female and the specific differences 

 recognised; but there are some in which the sexes are so much 

 alike that it is almost impossible to tell one from the other. In 

 others, again, the sexes are so different — as, for instance, in the 

 Arrhenuri — that one would be disinclined to think they could 

 be of the same species. 



The three figures are intended to convey to the beginner 

 the three stages mentioned. Fig. 1 is the larva of Piuna nodatiiss 

 (Mull.). Fig. 2, the nymph of Dip/ndnnlun despicievs (Mull.), 

 showing the ventral surface and the epimeral plates to which 

 the eight legs are attached. Fig. 3 is the adult Dijdodontuf: 

 drspiciens (Mull.), ventral surface of female. 



'I'here is another point in the adult stage to which it will be 

 well to draw atteution. When the mite has first made its 



^-^=^^ 



Fig. I. 



appearance from the inert period it spends between the nymph 



and adult stage, the bard and chitinous parts appear to be nearly 



fully developed, but the soft parts are not so. The body 



often appears very small, while the 



palpi, legs, and epimera, etc., arc 



very large in proportion ; it is also 



very poor in colour. It would be well 



to ascertain that the mites are quite 



developed before making drawings 



and taking measurements. In my 



ignorance, when I first began the 



study of water-mites, I had to discard 



a number of drawings I had made 



of different specimens because they 



afterwards proved to be only different 



stages of growth of the same species j- ., 



of mite. 



For coUecting mites there is no better apparatus than the 

 usual collecting stick used by pond hunters, having a metal ring 

 attachable at its end which carries a cone-shaped net made of 

 silk or muslin, with a glass tube at the bottom. The advantage 

 of the tube is that the contents can be examined with an 



