AND OTHER INJURIOUS INSKCTS OF I907 AND I908. r8l 



Alacrosiplnini: and the short spine-hke terminal hairs of the spur, 

 found in all .^phids. In Pemphigus vagahundus tliese terminal hairs 

 are scattered on the spur, and have an oval sensorial surface at their 

 base. 



The sensoria are organs of special sense, which may be divided 

 into three classes according to form and position. First, the terminal 

 sensoria, present in all Aphididx, located laterally and singly at the 

 distal ends of the penultimate and ante-penultimate joints. Where 

 there are only three joints the third is the only one with a terminal 

 sensorium. These sensoria are large, circular and fringed with a row 

 of fine hairs. Inside the row of hairs there is a groove which marks 

 ofif the raised membrane. In some species of Phylloxera the terminal 

 sensorium of the third joint is long and oval and extends nearlv the 

 whole length of the joint. Second, the circular sensoria which belong 

 typically to the third joint. These are circular, usually smaller than 

 the terminal sensoria, sometimes but not always fringed with hairs. 

 In the PemphigiiKV and Schizoneiiriiur the circular sensoria are modi- 

 fied respectively into transverse and annular sensoria. The transverse 

 sensoria are long, oval in outline, and run traverselv across the joint. 

 The annular sensoria form a narrow ring around the joint. Third, the 

 marginal sensoria, which are small sensoria grouped around the ter- 

 minal sensorium of the last joint. The most primitive arrangement 

 is in Laehiius, where they are scattered or in scattered groups near the 

 terminal sensorium. In other genera they are arranged in a single 

 row beside the terminal sensorium, or grouped in a peculiar pyramidal 

 arrangement beside it. A peculiar condition is found in Drepanosi- 

 phitm and Mastopoda, where two of the marginal sensoria are enlarged 

 and located one at each end of the terminal sensorium ; while a row of 

 four smaller ones occupies the usual position beside it. The marginal 

 sensoria are fringed in this case. At the end of the second antennal 

 joint there is a minute sensorium which resembles these marginal sen- 

 soria closely. 



Besides these types there are also fused sensoria, which are of 

 two kinds ; those formed by the fusion of sensoria of the same type, 

 and those formed b}' the fusion of sensoria of dififerent types. The first 

 is formed by the fusion of either circular, transverse, or annular sen- 

 soria. The second by the fusion of one or two transverse or annular 

 sensoria with a terminal sensorium. 



The joints of the antenna are usually imbricated. Sometimes 

 these imbrications appear under high power as rows of short sharp 

 points forming regular patterns. Then again the points seem to have 

 united to form sharp ridges facing the distal end of the joint. In the 



