250 The Mechajjism of EvoLUTioiir in Leptinotaesa 



Primary biotype 7 — Occupies a central position in the series. The central 

 system of spots is always fused into a broadly open V, of which none of the ele- 

 ments ever becomes free. C is always free and distinct, and d, e, f group are 

 fused but never united to c or medianward to the middle group. The anterior 

 marginal spots are always absent and also the posterior marginal. This biotype 

 exhibits as little fluctuation as any excepting 1, and shows only increase or 

 decrease. (Fig. 35, 7.) It is the pivotal pattern type of the whole array, and 

 by adding to or taking from it almost any combination can be built up. It is 

 also a common one in nature. 



Primary liotype 8 — This comes from biotype 7 by the breaking up of the 



groups a' + V ^ ^ a+h and the d+e+f group into the component centers, 



figure 35, 8. This is a variable biotype. The anterior marginal spots and 

 posterior marginal spots are never present. G is always free and the central 

 and lateral areas never united. Spot d may extend anteriorly towards &, but 

 rarely fuses therewith. 



Primary biotype 9 — Shows a sharp change in the position and character of the 

 a spots where they are parallel with the median line and not widely divergent 

 anteriorly as in all the preceding types. This is accomplished by the median- 

 ward movement of the anterior center in spot a^ and a cephalward elongation of 

 the spot beyond the b spots, giving a more or less squared appearance of the 

 central system. The d, e, f group are united and d is often fused with b; e and c 

 send prolongations towards each other, which rarely meet. This results in giv- 

 ing an anterior-posterior striping appearance to the pattern, as shown in figure 

 35, 9. The anterior and posterior marginals are always absent and there is 

 never lateralward fusion between the central and lateral groups. 



Primary biotype 10 — ^Kepresents a divergent line from 9, in which the same 

 type of pattern persists, but is heavier, the fusion being always marked and 

 heavy, and the anterior marginal spots being present and fused with a, forming 

 a prong projecting medianward. This type is represented in figure 35, 10, as 

 a quite distinct but not common one in the population. 



Primary biotype 11 — Differs from biotypes 9 and 10 by the absence of all 

 marginal spots, the fact that a, b, and c never fu^e, and the tendency to fuse 

 between d and e, which do not fuse with either a, b, or c. (Fig. 35, 11.) 



Primary biotype 12 — ^This is comparable to No. 8, in that it represents a case 

 in which all centers are free, the only fusion being produced by somatic vari- 

 ations. A tendency to restrict the pigment formation to the color centers also 

 accompanies this biotype. Figure 35, 13, shows the usual range of variation 

 of this group. 



These 18 biotypes of the pronotal pattern are based, as has appeared in the 

 above description, upon a system of color centers shown in figure 40, between 

 which certain combinations are made, and to the occurrence of certain marginal 

 spots which enter into the combinations. 



These biotypes overlap in their " variations," giving a complete intergradiag 

 series in any population, and all interbreed freely when present, so that the 

 natural population is exceedingly complex with respect to this character. These 

 primary biotypes may easily be entirely isolated and manipulated as " units," but 

 are to a greater or less extent associated with other characters or groups, form- 



