Brain and metamorphosis of lepidoptera

Abstract

Metamorphosis of lepidopterans is, as in many other insects, under the control of hormones coming from three different sources, the prothoracic glands, the corpora allata, and the brain. The prothoracic gland hormone is directly responsible for metamorphosis while the corpus allatum hormone can act as a block to the event. The brain hormone stimulates the prothoracic glands. In the normal course of development in Philosamia and other nondiapausing species, the corpora allata decline in activity late in the larval stage, and thus allow the caterpillar to metamorphose into a pupa and the pupa into an adult by the action of the prothoracic gland hormone, secretion of which is epistatically insured by the brain hormone. Since the brain hormone is prothoracotropic in nature, extirpation of the brain causes the prothoracic glands to lose the secretory activity. Thus for instance, the brainless pupa cannot continue further development due to lack of the prothoracic gland hormone, and consequently is forced to enter an artificial diapause. If, however, active brains are implanted into these brainless pupae, activity of the prothoracic glands to secrete the prothoracic gland hormone is stimulated and eventually the pupae differentiate into the adults. Natural diapause that occurs in some species such as Luehdorfia japonica and Papilio xuthus in the autumn form is also clue to a temporary failure in the secretion of the brain hormone and therefore it can also be terminated by supplying active brains. It is of interest that, during diapause the prothoracic glands can respond to the brain hormone, and that the tissues are also competent to initiate adult development at any time whenever the prothoracic gland hormone is provided. In short, the brain hormone is most important in the pupal diapause as well as in the metamorphosis of lepidopterous insects. The corpus allatum serves as a storage organ for this brain hormone in addition to its other function of producing the juvenile hormone. This is proved not only by the histological and electron microscopic observations but also by the experiment of implanting corpora allata into brainless pupae with a resultant molting which may or may not be coupled with adult differentiation depending on the activity of the organs. It is an intricate and not vet settled problem that the same phenomena of development can be induced in the isolated abdomen deprived of the prothoracic glands. A tentative explanation for this is that the brain hormone may be a kind of precursor of the prothoraeic gland hormone. To substantiate this assumption, the extraction of the brain hormone is now being performed. Preliminary studies indicate that it is water soluble and stable at 90°C for 90 min; it remains active in a deep-freezer at −18°C for 21 days or after desiccation over silica gel for 1 day.