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of the broods generally occurs, especially in the case of eggs laid by beetles 

 emerging from the original food-plant, in which case the mortality 

 is often total. Half- to full-grown larvae, however, usually can be 

 successfully transferred to a new food-plant and live and transform 

 to adults. With some species that can be reared in a secondary (new) 

 food-plant, by the larvae feeding one or part of one year, preference 

 for that food-plant is shown by the resulting adults. In general, the 

 fewer the food-plants in nature, the more marked the predilection 

 for a particular food-plant, and vice versa. Continued breeding in 

 a given food-plant intensiiies the preference for that food-plant. The 

 condition of the food-plant has a great influence on food-plant selection, 

 in that every species prefers an optimum condition of the food-plant 

 which it selects and will choose a new food-plant in the optimum 

 condition in preference to an old food-plant in which the conditions 

 are unfavourable. The quantity of wood at the disposal of the ovi- 

 positing adults may influence the insects in their choice between 

 different kinds of host wood, in that, if there are many adults to a 

 limited amount of the primary food-plant, some species will select a 

 secondary food-plant if such is available. If this is done, however, 

 the resulting brood is weakened. It is altogether possible that these 

 experiments may indicate the origin of certain closely related species 

 or varieties of insects. For instance, a species restricted to a very few 

 plants may accidentally be forced to take a new food-plant (as actually 

 happened in the experiments with Cyllene in oak). A few individuals 

 may survive and continue the strain so that it becomes, after a time, 

 at least physiologically different and may also develop correlated 

 differences of colour or structure. It can hardly be said that such 

 forms are much less distinct than in the case of the two species Callidiiim 

 antennatiim in pine and C.janthimim in juniper ; for even though these 

 have a slight colour distinction, and each is absolutely restricted to 

 its own food-plants, they interbreed. On the other hand, in the 

 different forms of Hylotnipes ligneits, of which the eastern form in 

 juniper is constant in marking, the w^estern form in redwood is quite 

 variable, as is also the Rocky Mountain form in Douglas fir. The 

 juniper and redwood forms interbreed, but all attempts to mate 

 either of these with the Douglas fir form have failed. All these forms 

 can be furnished with substitute food-plants, but in the experiments 

 in which this has been done the original colour pattern has resulted 

 thus far. 



The grape and hickory strains of Cyllene pictus, although showing 

 no colour differences, do not readily mate. Two species of Cyllene, 

 C. pictus and C. robiniae, are separable only as adults, by a slight 

 difference in the colour pattern, yet in seasonal and biological habits 

 they are strikingly different. It is conceivable that one of the two 

 species originated through the adoption of a new plant and continuous 

 breeding in that plant. 



That the change of food-plant in nature is not of more common 

 occurrence is believed to be due to the high mortaUty of the first stage 

 larvae in a new food-plant rather than to the absence of oviposition 

 in the new food-plant. Although the adults show a decided pre- 

 dilection for a favoured food-plant in ovipositing and even, in certain 

 species, a preference for the plants in which the larvae have fed for 

 one or two generations, the instinct to oviposit seems to overbalance 

 that of host selection, consequently new food-plants are frequently 

 selected, possibly more frequently in nature than is generally reaUsed. 



