INTRODUCTION. I7 



to the extremity of the abdomen, is sixty-seven millime- 

 ters, or, omitting the antennae, fifty-two millimeters; the 

 expanse of wings one hundred and seventeen millimeters, 

 or a little more than four and one-half inches. The 

 smallest dipteron that I have ever observed in the 

 examination of many thousand specimens and five 

 or six thousand species, is that of a cecidomyid measur- 

 ing a trifle less than one half millimeter, also omitting 

 the antennas. In other words, the Mydas is more than 

 one million times the size of the cecidomyid. Possibly 

 there are still greater discrepancies between the largest 

 and smallest specimens of the order, but in all prob- 

 ability not much. The largest insect known is one 

 allied to the dragon flies, an extinct Devonian species 

 which measured about fifteen inches in length. As- 

 suming that the bodily proportions of the largest and 

 smallest hexapods are not unlike, the extremes of size or 

 weight are more than four hundred millions apart. In 

 no single family of diptera are the differences in size 

 anywhere nearly so great as those between the mydaid 

 and cecidomyid. Seldom do the differences in linear meas- 

 urements in any one family exceed ten fold. Among 

 other families of diptera the tipulids, asilids, and especial- 

 ly the pautophthalmids, often furnish examples of large 

 size, while the tabanids, syrphids and cyrtids have not a 

 few forms of considerable size. The largest of all the 

 Cyclorrhapha will be found among the Calypterae, while 

 the Acalypterae are rarely much above the average in 

 size, and many are small, or very small. 



Giantism in any group of animal life is a specialization, 

 and is, in general, an indication of approaching decadence; 

 enduringly small races are never the descendants of 

 giants, for decrease in size means lessened vitality and 

 incipient extinction. No strong or dominant group of 

 flies, like the Tachinidae, Dolichopodidae, Syrphidae, or 



