The objects of the investigation were to describe the process of maturation of the ovaries, to clear up the question of the relation between copulation and sexual maturity, and to determine whether any relation exists between weight and sexual maturity.

The locusts used were of the transiens phase. They were kept at temperatures between 32°C. and 37°C. and relative humidities between 50 per cent, and 70 per cent.

The maturation of the ovaries was followed, and is divided into four stages distinguished chiefly by the sizes of the eggs.

It is shown that the average number of ovarioles per female does not change significantly with age.

The numbers of ovarioles in the two ovaries of an individual often shows asymmetry, which probably increases with age.

Some of the ovarioles are small. The percentage small ovarioles increases with development of the eggs. Variation between individuals in the total number of ovarioles also increases with age.

The number of egg rudiments per ovariole was calculated for different ages and stages. The differences were not significant. It is concluded that new rudiments are produced during the oviposition period.

It was found that when the food supply was relatively short, female locusts became mature and oviposited if allowed to pair, but did not mature if males were not present. With abundant food, unmated females matured, but more slowly than mated ones and fecundity was reduced. It is concluded that under unfavourable conditions pairing may be necessary for maturation of the ovaries.

Average weights of female locusts in each of the four stages were compared. Locusts in stages III and IV, i.e., those which are mature, were found to be significantly heavier than those in stages I and II. The average weight of a number of stage I locusts 13–18 days old (whose development had been retarded by unsuitable conditions) was also found to be significantly less than that of a number of stage III locusts of the same age. It is concluded that weight depends primarily on the development of the ovaries.

The relation between weight and age was investigated. Rate of increase of weight is rapid at first, is reduced for about two days, then increases again and maintains its value until the weight is about 2 gms. During the oviposition period it fluctuates irregularly.

It is shown that if conditions are unfavourable to maturation the weight rises to a certain value, after which it remains constant, or falls. The fall was best seen in stage II locusts over 14 days old.

There is little relation between weight and egg-size in stages I and II, but in stages III and IV the relation is closer. Rate of increase of weight with egg-size is at first low, but above an egg-length of about 4·5 mm. the rate of increase of weight becomes greater. It is suggested that this is because early growth of the eggs is due to transference of fat from the fat body and later stages to the laying down in the eggs of substances derived directly from the food.

Estimates were made of water and fat content of locusts of various ages. Percentage water falls rather rapidly in the first few days of adult life after which it remains approximately constant. Percentage fat rises to a maximum at about 14 days, then falls slowly. Most of the increase in weight during maturation is due to water intake.