578 A CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



velopment of social organization and the forces acting to cause such develop- 

 ment. Further comparative studies will shed a flood of light on the evolution of 

 instincts. Morphological studies of many sorts will provide further information 

 on phylogeny, which is needed to verify or alter the present classification. 

 Studies of such matters as parallelisms, orthogenesis, and the like can then be 

 approached on a sounder basis. Biosystematic studies of all groups will add 

 to our general knowledge of bee species, their ecologies, and their evolutionary 

 and distributional patterns. In this connection a matter of special interest 

 concerns pollen-collecting habits. Many species, termed polylectic, collect 

 pollen from all sources; others, known as oligolectic, from only a few related 

 species of plants. The evolution of this specialization, or the general problem 

 of host specificity, can well be studied among bees for every intergrade between 

 oligolecty and polylecty exists within numerous genera. These are merely some 

 of the biological problems upon which bees may well provide information and 

 upon which the present author and his students and associates hope to work. 

 We trust that others will help, for there is work enough for many. Some will 

 prefer to work on quite different problems, for example, pollination, sense physi- 

 ology, and so forth. 



One of the great troubles with most entomological papers is that they are 

 written to be read by only a very few specialists. They provide a mass of minutae 

 and few generalizations. Let us hope that more and more entomologists will 

 attack and solve, through the insect groups in which they specialize, problems 

 of general biological interest. Too many gather the needed data and are con- 

 tent to publish them without analysis, ignorant of or indifferent to the biologi- 

 cal principles to which these data may contribute. 



DIPTERA 



Charles P. Alexander 

 University of Massachusetts, Amherst 



As at present known, the Diptera or two-winged flies comprise the fourth 

 largest order of insects, with approximately 85,000 described species, which pos- 

 sibly is not more than some 20 per cent of the total number in existence. Some 

 of the better known countries and states have species of Diptera about as fol- 

 lows: Great Britain, 5,200; United States, 16,700; New York, 3,615; New Eng- 

 land, 3,325; Michigan, 3,235. 



Various classifications of the order have been proposed, the most recent by 

 Hennig (1948) which separates the Diptera into two suborders, Nematocera, 

 with sections Bibiomorpha and Culicomorpha, and Brachycera, with sections Ta- 

 banomorpha and Muscomorpha. A widely accepted arrangement, which is fol- 

 lowed in this paper, divides the order into two suborders, the Orthorrhapha with 

 two series, Nematocera and Brachycera, and the Cyclorrhapha with three series, 

 the Aschiza, Schizophora, and Pupipara. In the following brief account, the 

 leading events and many of the outstanding workei-s are indicated, together with 



