November — Decemher 1S90.] 



PSYCHE. 



413 



or sensory system attains its most com- 

 plete development, so far as my obser- 

 vations extend. Every joint, except 

 the small basal knobs, is imbricated, 

 and each is furnished with sensory pits 

 or pores. The third joint is irregular 

 and knobby from the projecting margins 

 of the pits, which are numerous and 

 very large. The figure shows the ap- 

 pearance well, but it must be said that 

 the enlargement of <5, is only § that of 

 a, c, (/, and ^, so that it is really equal 

 to the two following joints in lengtli. 

 Joint four has also a large number of 

 pits, irregularly scatt-ered on all sides, 

 and this is the more remarkable since 

 in none of the other species is it pitted 

 at all. Joint five has four large and 

 two small pits on one side — other not 

 examined — , more also than in the 

 other species. On joint six, on the 

 contrary, the aggregation of pits is smal- 

 ler than in the Cherry aphid, though 

 the number of small pits is greater. 

 Curiously enough, nothing in the larval 

 antennae indicates this extreme develop- 

 ment in the mature form. AH of the 

 wingless forms have this simple, single 

 pit near the tip of 5, and the usual 

 small group on 6. It has been a^lready 

 indicated that in this species the struc- 

 ture is best made out. This is due to 

 the fact that ever}' part is more thorough- 

 ly chitinized than in any other species 

 save M. cerasi, and the action of 

 potash and carbolic acid is not so de- 

 structive, while the parts are sufficiently 

 cleared for study. We see here that 

 we do not have simple pits to deal 

 with ; but rather special sense organs. 



apparently not directly communicating 

 with the outer air, for on careful exam- 

 ination a fine, tense membrane is seen 

 to close the opening, not from edge to 

 edge, but as if drawn over a projecting 

 rim. It is on a side view that we best 

 get an idea of the structure which is 

 fairly well shown at f and g-. What 

 are the functions of these pits.^ Not 

 tactile, surely! Olfactory.? Why should 

 that require any tense membrane.? Au- 

 ditory.? Here the membrane might 

 serve as a drum to catch the vibrations ; 

 but why should there be such a differ- 

 erence in number.? Why also do the 

 darker, more chitinized forms have 

 these pits relatively so much larger.? 

 These questions are easily asked ; but 

 I shall not try to answer them. 



Besides the antennae I also studied 

 the mouth parts of these insects and 

 find that there is some small difierence 

 in the structure here, shown in the fig- 

 ures given ; but scarcely worthy of 

 note, except in the case of the wheat 

 louse. In this species the beak is per- 

 haps shorter and broader than in the 

 others, while the ante-apical segment 

 has on each side a peculiar flap, gouged 

 from the body of the joint and covering 

 loosely the pit thus caused. This is 

 found in all stages of this species, and 

 nothing like it has been found in any 

 other examined by me. 



Within this beak are the four bristles, 

 two of them connate except at base, 

 which are supposed to represent the 

 maxillae and mandibles, the beak itself 

 being a modified labium. I wish to 

 decidedly state my disbelief in any such 



