ROBERTSON FLOWERS & INSECTS UMBELLIFER^. 457 



Review of the Umbellifer^. 



In a general wav. insects increase in numbers as it becomes warmer, 

 and we may expect to find more species on the later forms. 



Flies are common throughout. They show a preponderance in the 

 lists of early species, except those with concealed nectar. They reach 

 their maximum proportion on Ileracleum and Eulophus. In Cicuta they 

 decrease on account of increased competition with hymenoptera, and in 

 Eryngium also on account of concealment of nectar. As far as the food 

 of the adults is concerned, the earlier months seem to be much more fa- 

 vorable to flower-flies. 



Bees also appear to be equally common throughout, only Zizia and 

 Pastinaca showing more species than Erigenia. The flowers with con- 

 cealed nectar and those blooming in early spring are most favorable to 

 them. 



Other families of hymenoptera show a marked increase in number of 

 species from early spring. x\ few forms appear as the season advances, 

 and they reach their maximum in July and August. The Eumenidas and 

 Crabronid?e are the only families in which the increase is not well 

 marked. We can best understand the changes in the lists if we regard 

 the bees and flies as practically stationary, their proportions and their 

 actual numbers in the lists being reduced by the increase in numbers and 

 the competition of the lower hymenoptera.* 



The character of the visitors of Umbelliferte. therefore, must depend 

 upon the insects to which they are exposed, i.e., upon the time of bloom- 

 ing. Indeed, the time of blooming is almost or quite as important as 

 modifications to hide the nectar. If we take three white-flowered forms 

 with exposed nectar, but appearing at difl'erent times, we will find the 

 extremes in character of visitors. Thus, Erigenia shows the highest 

 proportion of bees, Eulophus the highest proportion of flies, and Cicuta 

 the highest proportion of low-er hymenoptera. Zizia, Polytsenia and 

 Eryngium agree in showing a preponderance of hymenoptera over flies 

 as an eff'ect of concealment of honey. Pastinaca, Cicuta and Sium show 

 the same result as an tflFect of time of blooming. Forms with hidden 

 nectar also show marked contrasts on account of time of blooming. 

 Thus, in the case of Zizia. which blooms in May, one-half of the hymen- 

 optera are bees, while in Eryngium, which has more deeply-seated honey 

 but blooms in July, only one-third of the hymenoptera are bees. 



Concealment of nectar, however, accomplishes one important result 

 which cannot be accomplished by change of time of blooming, and that 

 is the simultaneous exclusion of flies and short-tongued hymenoptera. 

 The eff'ect of concealment of nectar can only be ascertained by compari- 

 son with a form having free honey, and blooming at the same time as 

 Eryngium with Cicuta. 



* See Table II. 



