Anthrenus verbasci (L.) (Col., Dermestidae) is a pest of dried animal materials and is widely distributed in temperate regions.

The effects of temperature and relative humidity on development have been studied. It is shown that the periods of incubation and pupation decrease with increase of temperature, the former from 54 days at 15°C. to 12 days at 30°C., and the latter from 89 days at 10°C. to 9 days at 25°C. Humidity differences have little effect.

The larval development is exceptional in that under constant physical conditions in the laboratory there is a rhythmical onset of the larval diapause, i.e., development is regulated into one or more cycles, each comprising a period of active growth followed by diapause. The length of this cycle under constant conditions is considerably less than a year. Larval development may extend over one or two cycles depending mainly upon the temperature. At low temperatures of 15°C., development extends over two cycles; at 20°C. and above, only one cycle is needed; at the intermediate temperature of 17·5°C., half the larvae require one and half require two cycles in which to complete their development.

When the larvae developed under constant physical conditions with malnutrition the rhythm of development was maintained, but the larvae tended to require an extra cycle for development compared with those fed on an adequate diet.

Under three sets of controlled alternating temperatures (ranging from 18 to 33°C.) the rhythm of development was similar to that which occurred in constant conditions at a temperature equal to the mean.

The number of larval moults increased both with temperature and the time spent as a larva. At the unfavourably high temperatures of 30 and 35°C., larvae moulted an excessive number of times; one larva at 30°C. moulted 19 times after the fully grown stage was reached; and, at 35°C., 30 per cent. R.H., 13 out of 20 larvae gradually decreased from about 5 mm. (the length when first put to 35°C.) to 2 mm., moulting a number of times in the process.

Under outdoor fluctuating conditions, the resting period in the cycle is extended, due to the winter temperatures prevailing for some time after the end of diapause. Active development commences again in the spring when the temperature rises, and in this way the cycle is synchronised with the seasons into an annual rhythm of development. The life-cycle under outdoor conditions may take one, two or more years to complete.

Field populations, developing both in attics of houses and under more outdoor conditions, pupated from January to May, the time of pupation being mainly dependent upon the ambient temperature prevailing after the termination of diapause.

Diapause is of value to A. verbasci because it induces a rhythm hi the life-cycle which synchronises with the rhythm of the seasons and ensures that the adults are present when the environment is favourable for their activity, i.e., during May and June when the maximum hours of sunshine favour flight and the preferred flowers are in bloom.

The mechanism by which the rhythmical onset of diapause is controlled has not been investigated.