June 1, 1S67.] 



HARDWICKE'S SCIENCE-GOSSIP. 



125 



elms of Dufour, the type of which is the little red 

 spider so injurious to plants in hothouses and 

 rooms, the Acarus telarius of Linngeus ; most of 

 the species live iu societies, on plants, and possess 

 the power of forming webs." Koch says, when 

 speaking of an allied species, Tetranychus socius, 

 " It appears in certain years in such numbers that 

 it covers the trunks and the branches of the lime- 

 trees which it frequents, with such a thick web that 

 they look as if clothed with glazed satin. I cannot 

 find any description of the species sent by you, 

 although it is closely allied to the common Tetrany- 

 chus telarius, and I uever before saw or found any- 

 thing like the curious nest which it inhabits." 

 Since the receipt of Mr. Meade's note, I have paid 

 some little attention to the Tetranychus telarius, 

 and find that the network of infinitely minute 

 silken threads is admirably adapted to its singu- 

 t'ormed feet, and these are equally well adapted. 

 to the office of holding on while it perforates the 

 cuticle of the leaf with its rostrum ; its hold is so 

 secure that no amount of washing by means of a 

 garden engine seems to have the effect of removing 

 it. As I have no doubt whatever that these little 

 creatures are exclusively vegetable feeders, the web 

 cannot serve, as in spiders, the purpose of securing 

 prey, and it is, moreover, never accompanied by the 

 glutinous particles which render the web of spiders 

 so adhesive. As a matter of course, if the Acari 

 caa resist the action of a water-engine, they have 

 little to fear from the effects of rain." 



Fig. 91. Tetranychus telarius. 



The "red spider," as it is called by gardeners, 

 having been alluded to iu the above communication, 

 as forming the type of the present genus, it has 

 been figured in this place, although too well known 

 to all who are associated with greenhouses and 

 conservatories (fig. ( JJ) to need further description. 



Ten years prior to the above communication by 

 Dr. Milner Barry, the plane-trees in Regent's Park 

 were observed by Air. George Wilson to be infested 

 by " sociable mites," of which occurrence the follow- 

 ing account was transmitted to the society already 

 named:— 



At the beginning of September, Air. Wilson's 

 attention was directed to the trees, several of 



which had the trunks and branches entirely or 

 partially covered with a very delicate web, upon 

 which myriads of a small Arachnidous insect were 

 running to and fro, extending their webs rapidly 

 along the branches. The web was so fine as to 

 appear like a thin compact layer of varnish upon 

 the stems of the trees, and from the vast number of 

 the insects, the grey web appeared dusted with a 

 reddish powder, the iusects being of a light orange 

 colour, inclining to brown. Prom the web so com- 

 pletely enveloping the tree aud obstructing the vital 

 influence of the atmosphere, the leaves became 

 withered and fell. This was especially the case 

 with the plane-trees, the elms and horse-chestnuts 

 being free from them. The weather for several days 

 previously and subsequently was fine and sultry, 

 but in the course of a few days a heavy fall of rain, 

 accompanied by a thunderstorm, put a stop to the 

 injury by destroying the insects. On placing a por- 

 tion of the web with its inhabitants in a bottle, Mr. 

 Wilson observed that in about an hour a beautiful 

 transparent cylinder had been spun within the 

 bottle, from the base to the top, impinging against 

 the side of the bottle at about half its height ; and it 

 was remarkable that there was not a single thread 

 stretched across the inside of the cylinder, nor was 

 a single insect enclosed within it. Having com- 

 pleted their first cylinder, they threw a second 

 around it, more slender than the former, leaving 

 only a small interval between them. 



Fig. 92. Tetranychus tiliarius. 



V 



Air. White considered the species to be Trom- 

 bidium tiliarum, or an allied species, and distinct 

 from Acarus telarius, L., and Trombidium socium, 

 the habits of which, as described by Hermann, were 

 mentioned by Air. White. 



The species described and figured by Hermann and 

 Koch under the name of Tetranychus tiliarius, or 

 the "Lime-tree Alite," is engraved (fig. 92) forthe 

 benefit of my readers. It must be observed that 

 bright colours characterize the majority of the 

 members of this genus, although that feature is 

 omitted in the woodcuts. 



Careful readers of the past volumes of Science- 



