i^^ AND ^"^/ij^ 



JOURNAL OF VARIATION. 



Vol. X. No. 10. October 15tii, 1898. 



Migration and Dispersal of Insects: Coccids and Aphides. 



By J. W. TUTT, F.E.S. 



So little is known of the dispersal and migration movements of the 

 Hemiptera, that one rather hesitates to deal with the matter at all, yet, 

 in the case of at least two groups, the Coccidae and A^yhididae, the 

 economic importance of a knowledge of their spread is so great, that 

 no other excuse is needed for very briefly referring to them. 



The Coccids and Aphidids both belong to the Homopterous group 

 of the Hemiptera, the former being popularly known as " scale " 

 insects, the latter as " plant-lice " or "green fly." The male Coccids, 

 unlike other Hemiptera, have a complete metamorphosis and usually 

 only a single pair of wings, the posterior wings being absent or reduced 

 to mere hooks. There arc, however, some exceptions to this, some 

 species never developing wings, and in Chionaspis sallcis, as discovered 

 by Newstcad, there are both apterous and winged forms, whilst in 

 Aiderococcus the male never develops wings. The female is always 

 wingless, usually grublike and stationary, and naked, as in Lecanium, 

 or covered by a waxy scale, powdery secretion, etc. Coccids that 

 secrete a covering that becomes hard and brittle, are well illustrated 

 by the commercial " lac " insect, Tachardia laccae, whilst the " soft " 

 scales, i.e., those that secrete a soft coating (the wax composing which 

 may be easily drawn out into cottony-looking threads) include the 

 genera Ptdvinaria, Dadylopius, etc. The species of the genus 

 Aspidiotus resemble the Apnididae, in being sometimes viviparous 

 and, as in the latter, successive broods are developed with amazing 

 rapidity. The Aleurodidae resemble Coccids in their immature (pupal) 

 condition, and, in that stage, are fixed to the plants ; in the adult stage, 

 however, both sexes are winged, and then resemble small plant-lice. 



The marvellous sedentary distribution of some Coccids suggests 

 that they must have some ready means of dispersal, but there can bo 

 no doubt that man has been mainly instrumental in the distribution 

 of others. This is exceedingly well shown by Green and Newstead, who 

 give (K.J/..V., xxxiii., pp. 08 rt. seq.) a list of the species found by 

 them in the Royal Gardens, at Kew, in July, 1896, where in one day 

 alone Green found 16 species, of which Newstead pronounced five to 

 bo new to Britain. These had evidently been imported, the various 

 species having been introduced from the United States, Madagascar, 

 Cuba, Trinidad, Malay, India, etc. 



Local migration of sedentary species can take place in the larval 

 state only, but this must bo necessarily of a very limited character, and 



