214 PROCEEDINGS OF THE NATIONAL MUSEUM. 



which, however, could not bo conipk^te without entailing- a decided 

 disadvantage to the species l)v hindering- its spread to other food plants. 

 Nature has established her line of equilibrium somewhere between the 

 two extremes, and we have a majorit}^ of short-winged individuals 

 favored by the absence of long Avings, l)ut yet in nearly every species 

 will be preserved in some sex, g-eneration, or individuals fully devel- 

 oped wings to assist in the" spreading of the species. This line of 

 "balance'' will 1)0 affected by nearly every habit of the species, so that 

 we may naturally expect to find it in dift'erent places in species having, 

 different habits, and such is indeed the case. (See p. 10.5.) 



Such a change of habit from frequenting an exposed to a protected 

 feeding ground would affect other organs than the wings. There 

 would no longer be any need of embedding the eggs for protection, 

 and should the atmosphere prove suflicienth' moist, they would 

 undoubtedly develop though laid upon the surface of the leaf or stem. |i 

 This would save much of the energy of oviposition, and in the course.' 

 of time the practice of embedding the eggs would cease altogether. \ 

 Having now no use for the ovipositor, that, too, would degenerate i 

 from disuse till, at most, a mere vestige would remain of this origi-i 

 nail}" well-developed organ. Some such course of development I be- 

 lieve to have taken place in the Phhpotliripidfe, and the chitinous rod 

 now found on the underside of the ninth a])d()minal segment just in 

 front of the sexual opening seems best explaina1)le as the remaining 

 vestige of the former ovipositor. (See Plate X, fig. 115.) As the 

 ovipositor ])ecame weaker and weaker otlxM- changes correlated to this 

 nuist have been in })rogress. The sheath which had contained the 

 ovipositor, being no longer needed, would naturally ])ecome closed up. 

 The ventral plates which had previously disappeared to provide room 

 for the sheath Avould not again develop, ])ut the edges of the dorsal 

 plates closing around still further would meet on the ventral line| 

 forming the tube of the Tubulifera. At the same time the sexual 

 opening seems to have moved backward till it reached the hind parti 

 of the ninth segment, where it is now found. 



Other modifications of the Prothysanopteron, found in the Tubulif- 

 era (mainly), may logically ])e traced to this one change of habit. Ij 

 refer to the trapezoidal form of the prothorax, the enlargement off 

 the fore legs, and the development of a tooth upon the fore tarsus 

 wliich thereby has lost one segment in a large number of forms, also 

 the flattened character of the ])ody, and possibly its elongation. 



In regard to the moditications of the prothorax and the fore pair of 

 legs, it is very evident that tiiey may all be related to the one simple 

 change of habit in regard to the place of feeding, which has beeni 

 assumed. Naturally considerable effort would frequently, perhaps ' 

 usually, })e required to drag their bodies through such narrow places 

 as those in which they lived. Any variation in the line of a more 



