236 Anna Maurizio 



These experiments further confirmed that pollens of dif- 

 ferent plants can vary greatly in their effect on the bees. 

 Thirty-five pollen samples were examined and these can be 

 divided into many different groups. Depending on their 

 biological effectiveness, they range from the very effective 

 (e.g. Salix spp., fruit trees, Papaver spp., Zea mays, Crocus 

 alhiflorus, Trifolium spp., Castanea sativa) to the almost 

 ineffective, e.g. all seven species of conifer examined (Pinus 

 cembra, P. silvestris, P, montana, Picea excelsa, Abies alba, 

 Cedrus libani, Pseudotsuga taxifolia). No relationship could 

 be established between the type of floral biology of any 

 particular plant and the effectiveness of its pollen on bees ; on 

 the other hand, pollen collected by bees w^as more effective 

 than hand-collected pollen from the same plant species. It 

 remains to be mentioned that bees are very selective in their 

 choice of pollen sources, and this, according to Louveaux 

 (1958), is connected with the nitrogen content of the pollen. 

 These observations on bees were recently confirmed by ex- 

 periments carried out with other insects (Tribolium larvae, 

 Koch, 1952; Osmia lignaria, Levin and Haydak, 1958). 

 These authors also established differences in effectiveness 

 between pollen from different plant species and between 

 hand and insect-collected pollen. 



Further evidence shows that the effect of pollen is time- 

 conditioned, i.e. consumption must take place within the 

 first 10-14 days of the bee's life in order to activate the 

 pharyngeal and wax glands as well as the fat body. Pollen- 

 feeding at a later stage has little or no effect (Beutler and 

 Opfinger, 1949; Freudenstein, 1958; de Groot, 1953; Kratky, 

 1931; Maurizio, 1954). 



Data obtained from the author's experiments were statistic- 

 ally interpreted in order to examine in greater detail the 

 relationship between lifespan and physiological condition. On 

 the basis of a multiple correlation (part-regression) it was 

 established that there is a positive relationship between 

 physiological condition and lifespan, i.e. that 56 per cent of 



