[Illustration FIG. 42.--Longitudinal section through the imago of Anosia plexippus, ♀, to show the internal anatomy: _t_, tongue; _p_, palpus; _a_, antenna; _pr_, prothorax; _mes_, mesothorax; _met_, metathorax; _ps_, pharyngeal sac; _b_, brain; _sog_, suboesophageal ganglion; 1-2, blended first and second ganglia of the larva; 3-4, blended third and fourth ganglia of the larva; _l_, _l_, _l_, the three legs; _ac_, aortal chamber; _dv_, dorsal vessel; _oe_, oesophagus; res, reservoir for air or food; _st_, stomach; _mv_, Malpighian vessels; _i_, intestine; _c_, colon; _r_, rectum; _cp_, copulatory pouch; _o_, oviduct; _ag_, accessory glands; _sp_, spermatheca; _ov_, ovaries (not fully developed); _nc_, nervous cord. (Magnified 3 diameters.) (Burgess.)]
_Polymorphism and Dimorphism._--Species of butterflies often show great differences in the different broods which appear. The brood which emerges in the springtime from the chrysalis, which has passed the winter under the snows, may differ very strikingly from the insect which appears in the second or summer brood; and the insects of the third or fall brood may differ again from either the spring or the summer brood. The careful student notes these differences. Such species are called polymorphic, that is, appearing under different forms. Some species reveal a singular difference between the sexes, and there may be two forms of the same sex in the same species. This is most common in the case of the female butterfly, and where there are two forms of the female or the male such a species is said to have dimorphic females or males. This phenomenon is revealed in the case of the well-known Turnus Butterfly; in the colder regions of the continent the females are yellow banded with black, like the males, but in more southern portions of the continent black females are quite common, and these dark females were once thought, before the truth was known, to constitute a separate species.
_Albinism and Melanism._--Albinos, white or light-colored forms, are quite common among butterflies, principally among the females. On the other hand, melanism, or a tendency to the production of dark or even black forms, reveals itself. Melanism is rather more common in the case of the male sex than in the female sex. The collector and student will always endeavor, if possible, to preserve these curious _aberrations_, as they are called. We do not yet entirely understand what are the causes which are at work to produce these changes in the color, and all such aberrant specimens have interest for the scientific man.