THE MIGRATION AND DISPERSAL OF INSECTS. 235 



Thoir movements appear to bo of two distinct kinds : (1) Local 

 movements, whicli take place regularly, from one food-plant to another 

 in spring and autumn. (2) General dispersal movements, when vast 

 bodies of winged Aphidids move from one district to another. 



Local migration of certain species of aphides takes place regularly, 

 especially in the case of those that live on herbaceous plants during 

 the summer. These species migrate regularly in autumn to a substi- 

 tute plant, which provides them with shelter, usually a woody plant or 

 tree, during the winter, and return to their summer pabulum in spring. 

 Thus the hop-aphis [i'Jiorodon Jiumnli), which sometimes causes such 

 great injury to the hop crop during the summer months, is reputed to 

 migrate in autumn to various species of Primus, growing in the near 

 neighbourhood. When the hop- vines mature and die, the aphides fly 

 to rrunus. Here the true males and females (Phorodon mahaleh) are 

 produced, and true eggs are laid. The eggs pass the winter in this 

 stage. In spring they hatch, giving birth to sexless aphides, for, 

 although they give birth to viviparous young, they can scarcely be called 

 females, since no corresponding male is known. The viviparous stem- 

 mothers go on producing young parthenogenetically, and after one or 

 two generations have been produced upon the plum, winged forms 

 develop and migrate back to the hop-plants for the summer. We have 

 it on Lichtenstein's authority that /'. mahaleh is the same insect as 

 P. humuli, the two representing only diiierent stages of its existence. 

 In America, there are parallel instances of a similar method of 

 migration.. The " melon louse " migrates from orange-trees to the 

 melon, another aphis (Ostri/a virginica) migrates between geranium 

 and hornbeam ; another from the common reed to the plum, whilst a 

 fourth passes from the milkweed (Asclepias) to the oak-tree. 



The Pemphigians or gall-making Aphidids, have also been shown 

 to migrate from one food-plant to another. Thus the gall-louse of 

 the f.entiscus, ApJoneura lentisci, passes, when it becomes winged, 

 to Bromus sterilis and Uordeum vidgare, and there deposits apterous 

 young, which feed on the grass roots during the winter, and in spring 

 become winged and return to the Lentiscus, where they deposit the 

 .sexuatcd pupjc. From these pupte both males and females, " without 

 rostrum," emerge ; these copulate, and the female lays a single egg, 

 from which proceeds the gall-maker, so that a Pomphigian presents 

 the following stages : (1) The '^egg" (only one laid by each female). 

 (2) The " fundatrix " (first larval state) which forms the gall, and 

 moults four times. (3) " The pseudogyne," which is vivigemmous, and 

 fills the gall with its proles. (1) The " emigrant pseudogyne " (second 

 larval state), winged and vivigemmous, which leaves the gall and flies 

 to gramineous plants. Those deposit (5) the " gemmantia " (third 

 larval state), which have an unlimited power of apterous reproduction 

 underground. From these, colonies of nymphs giving a winged form, 

 are developed. The winged form is (G) the "pupifora" (fourth larval 

 state), which carries back to and deposits on the Lentiscus the egg- 

 like pupa', from which the little apterous male and female lice emerge ; 

 these copulate, the female lays the fecundated egg, and the cycle is 

 completed (Lichtenstein). 



Kessler found the winged pupifera of Tetraneura ulmi, the well- 

 known elm gall-aphis, depositing on the elms the sexual forms 

 (without a rostrum), and later, Lichtenstein proved that the young of 



