152 The Mechanism of Evolution in Leptinotaesa 



LEPTINOTARSA DECEMLINEATA X LEPTINOTARSA OBLONGATA. 



The crossing of these two species has given further data concerning the 

 gametic comiDosition and reactions different from that derived in the crossing 

 of the other species described in Chapter IV. In Chapter II, I have described 

 these two species, and the chief points of difference, between them at all stages 

 in their ontogeny are shown. There is difference in the form and general type 

 that is measured by the form-index ; there are differences of color and pattern 

 in the larvae, in pattern and color of the adult, in reactions to different environ- 

 mental agents; but both have about the same rates of development {Ac values) 

 and feed upon species of succulent solanums ; L. oblongata upon S. rostratum 

 and its allies; L. decemlineata upon either the rostratum or tuberosum groups. 

 L. oblongata, however, does not thrive as well upon 8. tuberosum as it does upon 

 the 8. rostratum, and although it can be made to eat tuberosum, it always shows 

 a decided preference for its native food. Both are easily grown side by side in 

 the laboratory with perfect success, showing no changes as the result of the 

 enforced captivity, so that crosses can be made under the conditions of the 

 breeding-quarters, and thus to a large extent eliminate the possible complica- 

 tions that external conditions might introduce into the experiments. 



The crossing of the species is not a perfectly free one, although it is obtained 

 with relative ease in all of the stocks of these species that I have seen. Some 

 cross more freely than others, and all with greater certainty of success after they 

 have been under the same conditions for several generations and have been 

 subjected to the same routine breeding treatments. There is no entire sterility 

 between them, and infertility seems on the whole to be failure to complete 

 development rather than failure of the two to breed. All of the F^ hybrids are 

 fertile and have given fertile progeny, so that the operations are not compli- 

 cated by the problems of sterility or lack of interfertility of F^. It is true, how- 

 ever, that they are less certain to produce progeny when the stocks are fresh from 

 the original habitat than when both the lines have been long grown under the 

 same conditions. 



Under the conditions given as to source of stocks and conditions of experi- 

 ment, the crossing of these two species is always complicated by the extent to 

 which the cross shows dissociation of the two gametic systems and the number 

 of interchanges of elements that occur. This is evident even in F^, and in Fa 

 the complexity is realized more fully when one attempts to work out any of these 

 series of crosses. All of the crosses recorded here are between stocks of L. oblon- 

 gata that originated from the " Temisco colony" near Cuernavaca, Mexico 

 (No. 607 and No. 619), and my stock of L. decemlineata at Chicago (No. 99). 

 When crosses are made between these two species, in the F^ fraternities the 

 larvas show uniform dominance in color and pattern of the decemlineata parent, 

 but in the adults the fraternity shows several well-marked combinations of the 

 parental characters in widely varying ratios. The types commonly found in F^ 

 are as follows : 



{a) decemlineata in form, with pale yellow-white hypodermal color. 



(&) decemlineata in form, with pronotal color of decemlineata and yellow ely- 



tral color of oblongata. 

 (c) decemlineata in form, with yellow-white elytral color of decemlineata, and 



yellow pronotal color of oblongata. 



