TRANSACTIONS OF SECTION D. 751 



plant Louse does not hibernate underground on the roots of the hop ; nor in, on, or 

 about anything in the hop-yard ; but that, upon the advent of the first severe 

 frosts, the hop-plant and the hop-yards are entirely cleared of the species in any 

 form. I find that all statements to the contrary in America are based on misap- 

 prehension, or mistaken identity of species, and" I believe (though admitting the 

 possibility of variation in this respect in milder climates) that the same will be 

 found to hold true m England, wliere hibernation on the hop-root has been accepted 

 by high authority. The positive statements made about eggs being laid in autumn, 

 whether on roots or upon the bines left in cutting, or which are carted away, are 

 based on conjecture, and have been blindly copied without credit by one writer 

 from another, a practice too common among secondhand writers on economic ento- 



The conjectures of some of the best students of Aphidology that Phorodon 

 humidi had a form (mahaleb, Fonsc.) living on Prunus, and that there was a conse- 

 quent migration from one plant to the other, I have positively proved to be correct, 

 by direct colonising from Prunus to Humulus, and by continuous rearing from the 

 original stem-mother hatched from the winter egg. 



The observations have been made on growing plants and in vivaria at Washington, 

 and checked by others made simultaneously in hop-yards at Richfield Sprino-s, N.Y. 

 An incident may here be recorded as illustrating the efiect of meteorological extremes 

 upon aphides. The extreme heat (over 100° F.) and dryness of July 17th and 18th 

 killed every one of the insects under observation at Washington, entirely clearino- 

 the plants. The economic bearing of such exceptional phenomena, as also of the 

 biologic observations made, is readily understood. 



The more important conclusions from the studies so far made are thus summed 

 up in a paper which I had the honour to read before the American Association at 

 its recent meeting in New York : — 



1. rhorodon humuli hibernates in the winter-egg state, this e^g beino- fastened 

 to the twigs (generally the previous year's gi-owth) of difierent varieties and species 

 of Prunus, both wild and cultivated. The egg is ovoid and 0'04 mm. Ion"-, o-reen 

 when first laid, but polished black subsequently. 



2. The annual life-cycle is begun upon Prunus by the stem-mother, which 

 hatches from this winter-egg. This stem-mother is stouter than the individuals of 

 any of the other generations, with the legs, antennae, and honey-tubes relatively 

 shorter, while the cornicles between the antennre are sub-obsolete. The colour is 

 uniform pale green, with bright red eyes and faintly dusky tarsi. 



3. Three parthenogenetic generations are produced upon Prunus, the second at 

 once distinguished by its more elongate form, much longer members, distinct cor- 

 nicles, and markings of darker green ; while the third (or typical mahaleb form) 

 becomes winged, and instmctively abandons the Plum and migrates to Humulus. 

 The habit of moving from plant to plant after giving birth to an individual, and 

 thus scattering the germs of infection on Humulus, is well marked in this winded 

 generation. 



4. During the development of the three plum-feeding generations, the Hop is 

 always free and, subsequently, until the return migration, the Plum becomes more or 

 less fully free from infection bj^ this species. 



5. A number of parthenogenetic wingless generations are produced on the Hop 

 (seven, or the tenth from the stem-mother on Plum haying been (raced up to 

 August 5th, and advices of the eleventh up to August 19th having been received 

 since my arrival in England) ; and, finally, there is a return migration of wino-ed 

 females to the Plum in autumn. The wingless Hop generations are not only'^in- 

 capable of migrating to Plum, but do not thrive upon it when artificially transferred 

 thereto. 



6. Exact observations are not yet complete as to the fuU number of "-enerations 

 produced upon the Hop before the winged return migrant appears, and another 

 month's careful watching and experiment is needed to fill this hiatus in the annual 

 cycle, as also to ascertain the exact number of generations produced m autumn on 

 the Plum. From knowledge extant and previous general observation, the facts 

 will probably prove to be as follows : — 



