INVESTIGATIONS ON THE MUTILLID WASPS 25 



find that they are much more extensive and important than is n(jw 

 suspected. Mutillids then are to be regarded as injurious or bene- 

 ficial insects to man according to the points of view outlined above. 

 They are probably active in one or the other of these roles more 

 extensively than we think. 



EFFECT OF TEMPERATURE ON TH^ ACTIVITY OF MUTILLIDS 



In 1922 I was extremely fortunate in learning of a sand-dune 

 area north of Minneapolis, Minnesota, where Mutillids were ex- 

 ceedingly abundant. During the summer of that year I spent a 

 number of days collecting these insects, and it was apparent from 

 the first that the abundance of individuals varied at various times 

 of the day. For example, very few Mutillids could be found on the 

 sand dunes before 8 o'clock in the morning. The number of indi- 

 viduals increased after that time, and collecting was good until about 

 11.30. From this time until 2.30 in the afternoon Mutillids were 

 scarce, and then the numbers increased again, and collecting was 

 good until late in the afternoon. This fluctuation in the abundance 

 of individuals at various times of the day was thought to be due to 

 temperature, but no definite data were secured. 



Chapman and others (1926) studied this same sand-dune area and 

 the insects characteristic of it. They found that the physical con- 

 ditions of the environment on this sand-dune area were essentially 

 like those characteristic of deserts. The highest temperature record 

 obtained on the surface of the sand was 56° C. No winter tempera- 

 ture records were made, but temperatures as low as —40° C. have 

 been recorded for this vicinity. It was found that during the normal 

 course of the day all the insects leave the surface of the sand when 

 its temperature nears 50° C. In order to avoid this temperature, 

 some climb grasses and some enter their burrows, while others fly 

 about some distance above the sand, making hurried landings to enter 

 their burrows. The female Mutillids were consistently the last to 

 retreat when the temperature rose and the first to return to the open 

 sand when the temperature fell. On July 24, 1923, and August 2, 

 1923, observations were made early in the morning to determine the 

 time and temperature at which the various insects appeared upon 

 the sand. On July 24 the first Mutillid seen was a male at 5.45 a. m., 

 when the air temperature was 23° C. and the temperature of the 

 surface sand was 17° C. On August 2 the first Mutillid seen was 

 at 6.55 a. m., when the temperature was approximately 18° C. In a 

 laboratory experiment in which approximately eight female and 

 eight male Mutillids were used, and in which temperatures were con- 

 trolled and gradually raised from 10° C. to 60° C, the following 

 results were obtained: All were inactive at 10° C; the first move- 



