INVESTIGATIONS ON THE MUTILLID WASPS 



19 



85 cocoons of Bembix pruinosa I obtained one male and three females of the 

 same Mutillid ranging in size from 11 to 15 mm. When a recently emerged 

 female of the latter group was placed with a male of the small series they 

 mated immediately. The specimens taken in copulation in the field had the 

 following length measurements: Female, 6.5 mm. and male 11.5 mm. 



The adults of Microbembex monodonta vary in size from 8 to 14 mm. while 

 those of Bembix pruinosa vary between 16 and 19 mm. The same propor- 

 tional differences exist in the size of the mature larvae of the two species. 

 The larvae of DasymuUlla bioculata parasitize the cocoons of both the small 

 Microbembex and the large Bembix and entirely consume the Bembecid larvae 

 before they enter the prepupal stage. The specimens of bioculata emerging 

 from the cocoons of the small Microbembex vary in size from 6.5 to 10 mm., 

 while those which emerge from the large Bembix vary in size from 11 to 15 

 nun. Inasmuch as tempera- 

 ture, humidity, light, quality 

 of food, and other ecological 

 factors vary uniformly in this 

 sand dune area, while we know 

 that the quantity of food is 

 considerably less in the Micro- 

 bembex cocoons which produce 

 the small bioculata than the 

 amomit of food in the Bembix 

 cocoons which produce large 

 bioculata, it seems justifiable 

 and reasonable to conclude 

 that the quantity of food avail- 

 able to the larvae of Dasymu- 

 tilla bioculata is the factor 

 which determines the size that 

 the adult will be. The quan- 

 tity of food may be either large 

 or small according to the spe- 

 cies of Bembicid which the 

 Mutillid parasitizes. 



As stated previously, when a 

 curve is plotted representing 

 the frequency and range of the 

 variation in size of a large number of specimens of Dasymutilla bioculata the 

 curve will be found to be of the bimodal type. The first mode will represent 

 the specimens which as larvae were parasites of Microbembex monodonta Say, 

 and the second mode will represent the specimens which as larvae were para- 

 sites of Bembix pruinosa Fox. Thus it is clear that in this case at least, what 

 appears from an examination of museum specimens to be a specific difference 

 in size is in reality an ecological variation of the bimodal type, due to a differ- 

 ence in the quantity of the food supply. 



Fig. 2. — Coreelation in size between Dasymu- 

 tilla BIOCULATA CRESSON AND ITS HOSTS MI- 

 CROBEMBEX MONODONTA SAY (LEFT) AND BEMBIX 

 PRUINOSA Fox (right). IN EACH VERTICAL ROW! 



Male above, female in the middle, host 



BELOW 



ECONOMIC IMPORTANCE 



It can not be doubted that a group of insects so numerous m 

 species as the Mutillids must be of more or less economic importance. 

 The relationships which these wasps have with their numerous but 

 unknown hosts must finally reflect themselves in the relation of 

 insects to man although it may be somewhat indirect. 



