92 



"West Indian P. pergandei differs a little from tlie type form in its scale, 

 so that I have named it variety crotonis* 



In the Eastern States Asjndiotus iivw occurs on Vitis; at Las Cruces, 

 N. Mex., a slight variety, var. coloratus^] is found on Chilopsis, to which 

 it is quite destructive, but never ou Vitis. 



Conchaspis angrceci is a species observed in the West Indies only on 

 cultivated orchids; in Mexico Professor Townsend has found a variety 

 of it living on Hibiscus. | 



Numerous other examples might be cited in which marked differ- 

 ences in food habit coincide with varying degrees of apparent struc- 

 tural variations in the insects — ranging from inappreciable variations, 

 as in the Jamaican A. aurantii, to differences sufficient to separate 

 distinct species. 



In the latter case we see no cause for surprise, but it is certainly 

 worthy of our attention that certain slight varieties or indistinguish- 

 able colonies of individuals act as though they vt^ere distinct species. 

 Ordinarily our species are founded on structural characters; but these 

 are what I have called physiological species, easily distinguishable by 

 their habits, but hardly or not at all in their structure. That they 

 tend in the course of time to develop into ordinary species Avith dis- 

 tinct structural characters is of course more than probable. That 

 they already in every case possess some structural peculiarities too 

 subtle to be observed is not impossible or unlikely, in some 

 instances, as noted, there is just enough of difference to ])ermit the 

 separation of a variety. 



From an economic point of view the importance of these facts is mani- 

 fest. Are we to suppose that the Jamaican Aspidiotus a urantii, so called, 

 is really a physiological species distinct from Californian aurantii f If 

 so, would California aurantii, if introduced into Jamaica, proceed to 

 play havoc with the citrus trees? There is room for some experimental 

 work here. Is it possible that aurantii can not exist on Jamaican 

 citrus trees because of the abundance of other species, particularly 

 articulatus f 



W^e can understand why the Chilopsis Aspidiotus in New Mexico does 

 not attack the grape-vines (which are there free from coccids), since 

 there is a slight difference in the insect. But it is so slight that it 

 would assuredly have escaped notice but for the accompanying physio- 

 logical peculiarities. 



Another instance is off'ered by the common large species of Pulvi- 

 naria found in this country. They are so much alike in structure that 

 they all go, as a rule, under the name of P. innumerabilis. The osage 

 orange form, P. machirw, is, I am convinced, a distinct species; and 

 probably a tliorough study, with abundant material from all parts of the 

 country, would reveal several i^hysiological species or varieties. To sift 



* Ann. Mag. Nat. Hist., July, 1895, page 62. 



t Psyche, 1893, page 571. 



t Bull. Bot. Dept., Jamaica, May, 1895, page 101. 



