WALNUT APHIDES IN CALIFORNIA. 23 



body, 0.754 mm.; wing expanse, 4.44 mm. Antenna, joint I, 0.076 mm.; joint II 

 0.060 mm.; joint III, 0.416 mm.; joint IV, 0.273 mm.; joint V, 0.273 mm.; joint VI, 

 0.143 mm.; filament, 0.164 mm. 



Described from many specimens taken at San Jose, Cal., during 

 1911 and 1912. 



The complete absence of the stigmatic vein and the relatively 

 longer antennae, together with the diminutive cornicles, will readily 

 distinguish this species from the European walnut aphid. 



In 18 months' study of this plant-louse the author has failed to 

 find any trace of the existence of a wingless viviparous form. 



The Oviparous or Sexual Forms. 



If a tree be heavily infested, the sexual forms appear first about the 

 middle of July and probably belong to the fifth and sixth generations. 

 If infestation be only moderate or slight, these forms are not pro- 

 duced until several weeks later and will be members of the seventh 

 and following generations. The sexed forms from the beginning are 

 produced in comparative abundance and comprise from 30 to 50 

 per cent of the whole. The young sexed females are paler and more 

 spindle-shaped than the young of the viviparous individuals, while 

 the male larvae and pupae are conspicuously brick-red in color. The 

 male is not so greatly outnumbered by the female as in the European 

 walnut aphis, and from the first comprises from 20 to 30 per cent of 

 the sexed insects. On August 26 and 27 and September 5, 1912, a 

 count of the forms on 34 leaflets taken at random from an Eastern 

 black walnut tree showed 177 viviparous females, 14 males, and 26 

 sexed females. Probably as many again of the sexed females in 

 proportion to the leaflets counted could be found on the twigs 

 ovipositing. Copulation takes place on the leaf and occupies half a 

 minute. All through August and September, 1912, the oviparous 

 females were observed on the twigs, but few eggs were found until 

 September. After the middle of September few aphides were found, 

 the great majority having been destroyed by their natural enemies, 

 but those that escape perpetuate the species until the leaves fall in 

 November. The majority of aphides born in the late fall are sexual. 

 The sexed female shortly after mating becomes much swollen by 

 reason of the growing ova in her body, and the last four abdominal 

 segments become orange colored. She repairs to the twigs and limbs 

 and wanders around searching for locations wherein to oviposit. 

 Occasionally immature females wander off to the twigs, but later 

 return to the leaves to resume feeding. The fully mature fertilized 

 oviparous female once she has forsaken the leaf rarely if ever returns, 

 and thus escapes many predatory foes. Having found a crevice or 

 crack in the cortex suitable for her purpose she grips the limb with 



