588 HEMIPTERA-HOMOPTERA CHAP. 



a bud or some growing part of the spruce-fir is affected in such a 

 way that it gives rise to an object having externally the appear- 

 ance of a little fir-cone, while inside it consists of chambers in which 

 the Aphids reside. The forms of Aphid-galls are very diverse, but 

 this is probably due to the plant rather than to the Insect, for the 

 same species of Aphis may give rise to different forms of galls. 

 Reaumur thought that each Aphid-gall was due to a single indi- 

 vidual that irritated the tissue of the plant, so that the latter 

 grew up at the point of irritation and enclosed the Insect. 



A few points as to the anatomy of Aphids should be 

 noticed. It is doubtful whether the antennae have ever really 

 more than six joints, the apparent seventh joint being actually a sort 

 of appendage of the sixth. The rostrum is externally three-jointed, 

 and is remarkable for the great diversity in its length, sometimes 

 it is quite short, at others several times longer than the body 

 (Fig. 285); the setae are often very much longer than the 

 sheath ; in cases where this great length of rostrum exists, the 

 individual may often be found with the tip firmly fixed in the 

 bark, and, as it were, tethered by means of the rostrum, the 

 length of which allows, nevertheless, considerable locomotion. 

 Suction is performed by contractions of the pharynx. There 

 has been much difference of opinion as to whether there is a 

 salivary syringe, and Witlaczil failed to find it. Krassilstschik is, 

 however, positive that it exists, 1 and that it is analogous to that 

 described by Mayer in Pyrrhocoris, but there are great differences 

 of structure between the two. It is very difficult to determine 

 the number of segments at the extremity of the body ; this is 

 terminated dorsally by a median organ placed above the anus, 

 and known as the cauda. Balbiani apparently considers that 

 there are ten abdominal segments and the cauda. The alimentary 

 canal has a small stomach, and an elongate intestine, the 

 terminal division of which is capacious and remarkably long. 

 There are no Malpighian tubes ; according to Kowalevsky, their 

 function is discharged by the posterior part of the alimentary 

 canal. There exists, however, a peculiar structure, the pseudo- 

 vitellus, a sort of cellular, double string ; and Witlaczil, in his 

 valuable paper 2 on the anatomy of Aphidae, suggests that this 



1 Zool. Anz. xv. 1892, p. 220. 



2 Arb. Inst. Wien, iv. 1882, Heft iii. \>. 397 ; see on this organ also Mordwilko, 

 Zool. Anz. xviii. 1895, p. 357. 



