32 Bulletin Wisconsin Nctfural History Society. [Vol. 7, Nos. 1-2. 



Number Alio- Hemi. 



Tube of tropous tropous Eutropous 



length visitors % % % 



Aster lateriflorus ... 1 mm. 86 52.3 44.2 3.5 



patliculatUS.. . . iy 4 mm. 92 39.1 55.4 5.5 



furcatus 1% mm. 86 25.6 58.1 16.3 



macrophyllus . . iy, mm. 95 33.7 49.5 16.8 



prenanthoides.. 1% mm. 64 34.4 56.2 9.4 



Drummondii. . . 1% mm. 81 25.9 61.7 12.4 



puniceus 3 mm. 77 24.7 59.7 15.6 



Imns 3 mm. 72 16.7 61.1 22.2 



novw-anglice . . . 4 mm. 46 6.5 63.0 30.5 



In Aster lateriflorus, the species with the shortest tube (i mm.), 

 the width equals the length, the tube is cup-shaped. Insects with 

 comparatively short tongues can reach the bottom, and access is 

 still more facilitated by the fact that the lobes of the corolla are 

 reflexed, and therefore do not stand in the way of the visitors. 



In A. paniculatus (length of tube ij4 mm., width ^4 mm.) the 

 tube is bell-shaped, and the corollar lobes are divergent. The 

 latter occupy a position intermediate between horizontal and erect, 

 and the same is the case in A. furcatus (i 1 /* mm. length) , A. mac- 

 rophyllus (1J/2 mm.), and A. prenanthoides (ij4 mm.). In the 

 remaining species the corollar lobes are erect, or even somewhat 

 inclined towards the middle, as in A. Drummondii, and such a 

 position of the lobes in these species with longer tubes strengthens 

 the effect of increase in tube length of restricting the visits of the 

 shorter tongues. As regards the latter a glance at the table above 

 shows that they are not entirely absent from the florets with long 

 tubes, Aster novw-angluv (tube length 4 mm.) for example still 

 showing a percentage of 6,5 of allotropous visitors. The Com- 

 positae of our flora possessing the greatest tube length are thistles 

 of the genus Cirsium, and in these we find the percentage of allo- 

 tropous insects amounting to 8,3 in C. lanceolaium, and 5,9 in C. 

 altissimum. These are as a rule pollen-eating beetles or flies, and 

 since in the Composite pollen is offered in abundance outside 

 of the tube, the greater length of the latter does not prevent such 

 insects from appearing at these flowers. But their number is 

 small, and it is a well known fact that the more deep-seated the 

 nectar the smaller in general the number of visitors as a result of 

 the exclusion of the shorter tongued species. This is demon- 

 strated very clearly by the following table, in which the 37 species 

 of Compositse under discussion are arranged in 6 groups accord- 

 ing to tube length, and the average number of visitors per species 

 is given. 



