58 MR. STANLEY HIRST ON THE 



Genus Oligonychus* Berlese, 1886. 



It is cliiRcult to separate the species of Oligonychtcs by means 

 of a key. The principal differences between the species of this 

 genus are in the structure of the penis and terminal finger of the 

 palp (see accompanying figures). 



1. Oligonychus ulmi 0. L. Koch, 1835. 



Tetranychtis ulmi C. L. Koch, Deutsch. Crust,, Myr., Arachn. 

 1825, heft ],no. 11. 



T. pilosus Qa,Yies>ti\i\i & Fanzago, Atti Soc. Yen. Trent. 1876, 

 V. pp. 133-134. 



T. ulmi Berlese, Acari dann. piant. coltiv. 1886, p. 22. 



T. pilosus ^evleae, Acari etc, in Ital, reperta, 1889, fasc, Ivi, 

 no, 6, 



Parateiranychus pilosus Zacher, Berlin Mitt, biol. Anst. 1913, 

 heft 14, pp. 38-39, text-fig. 1, 



P. pilosns Triigardh, Stockholm Medd, Oentralanst. Fbrsoksv. 

 Jordbruksomr. 1915, vol. 109, no. 20, pp. 21-29 etc, text-figs. 



cJ , Penis strongly curved, being shaped as figured. 



Palp. Terminal finger short, being less than half the length of 

 the two neighbouring rod -like seta3 near it. Dorsal sensory linger 

 also short, 



$ , Palp. Terminal finger quite short but very stout, being 

 much shorter than the rod-like setae near it. Dorsal sensory 

 finger short and slender. 



Another chra-acteristic point is that the hairs of the body in 

 this species spring from slight tubercles. 



Colour. Dorsum dark reddish at the sides, and visually with a 

 paler reddish central band, the anterior end of the body also pale 

 red. Yery minute whitish specks arranged in longitudinal 

 series (one at the base of each hair) are also present on the 

 dorsum, the hairs themselves being pale ; appendages flavous. 

 Some specimens are dark (almost blackish), brown, and the central 

 pale reddish band may be absent. 



Eggs reddish. 



Measurements. S , length 260-310 jm ; 5 , 380-470 /a. 



Host plants, etc. This species is frequently met with on elms 

 in this country, causing a characteristic discoloration of the 

 leaves. It is obviously the species described by C. L. Koch 

 under the name tilmi, as being not uncommon on that tree in 

 Regensburg. His figure is quite good and recognisable, and it 

 is a great pity that the later name pilosus has been used instead 

 of his n.ame. The species sometimes attacks roses, especially 

 rambler roses, discolouring the leaves greatly and obviously 

 weakening the plants. "Various fruit trees — such as pear-trees, 

 apple-trees, plum-trees, etc. — are also affected by this species, but 

 the damage is usually very slight. It also occurs on the service 



* The name Taratetrani/chus has been used by recent authors for this genus, 

 but, as Banks points out (Ent. News Philad. 1917, vol. xxviii. p. 197), it should 

 be replaced b}' the earlier, name OUg any elms. 



