70 ANIMAL AGGREGATIONS 



reappear on several successive nights. A swarming female is soon 

 surrounded by several males. These swim rapidly in narrow circles 

 about her. In a little while they begin to shed sperm^ probably in 

 reaction to some secretion from the female, rendering the water 

 milky. Soon the female begins to shed her eggs, shrinking in bulk as 

 she does so, until, a shadow of her former self, she sinks through the 

 water to die. Lillie and Just, following a lead from Hempelmann 

 (191 1), assume that the maturing of the animals is dependent on 

 some relation of the life-history to the phases of the moon, involving, 

 probably, through lunar tidal variations, rhythmical alterations of 

 conditions of nutrition. 



HIBERNATION 



Over- wintering aggregations of animals have long been known. 

 This phenomenon in social bees has been noted in scientific litera- 

 ture for almost two hundred years (Reaumur, 1734-42). Barkow 

 (1846), in his monograph on hibernation written over three-quarters 

 of a century ago, has a short chapter in which he calls attention to 

 the winter aggregations of lepidopterous larvae, adult ants, bees, 

 true bugs, beetles, including the frequently observed case of the 

 coccinellid beetles, carp and the eel-like Muraena anguilla, snakes, 

 frogs, and a few mammals, including marmots and bats. Barkow 

 advances no theory to account for the congregation of these animals 

 but does state that there is a suggestion current that the animals 

 come together as a result of response to their sense of smell. 



This list of over-wintering aggregations has since been much ex- 

 tended, especially by Holmquist (1926), who has made extensive 

 studies on hibernating arthropods in the Chicago region. He reports 

 that of 329 identified species taken during the winter season, nearly 

 17 per cent were more or less closely aggregated. Omitting those 

 known to be of a somewhat social habit at other times of the year, 

 about 9 per cent of the species ordinarily solitary in the summer were 

 aggregated in winter. 



In the social bees careful experiments have shown that tempera- 

 ture-control results from such clusters (PhilHps and Demuth, 1914; 

 Phillips, 191 7); and Holmquist (1928) has demonstrated that protec- 



