ECOLOGICAL 217 



keep them, in the cold weather, in their own underground nests; 

 they guard them; they help in the rearing of each brood. 



A case of this sort has recently been examined by Eidmann. 

 He describes how, when the buds open in spring, the guardian 

 ants lead the aphids from the nest and on to suitable trees or 

 bushes. There the ants, of which there is one for each individual 

 aphis, or at least for each small group, mount guard over their 

 "cows", as Linnaeus called them, keeping all strangers at a dis- 

 tance. From time to time the ant "milks" the aphis and collects 

 the fluid, either to bring it to the nest or to pass it on to another 

 ant for this purpose. If the night turns cold the ant shepherds 

 the aphis back to the nest; but later in the summer, in warmer 

 weather, the ants retire alone, leaving their aphids on the plants 

 all night. In the morning the ants return and mount guard once 

 more, and by careful marking Eidmann was able to show that it 

 was always the same ant that returned to one aphis kept under 

 observation. If the ant was taken away another soon took 

 its place. 



As the summer grows in strength, and the prolific aphids become 

 more numerous, the traffic between the ants' nest and the plants, 

 the "pastures", becomes greater. Underground tunnels are con- 

 structed, or at least covered run-ways, extending even up the trunks 

 of the trees. These protected routes are only used during the day; 

 at night, although the traffic is greater, they are not needed and 

 are deserted. 



Eidmann took a census of an average-sized nest of the Black 

 Ant he studied, Lasius niger. He found in it nearly 3,500 adult 

 worker ants, and three times that number of larvae and pupae, 

 most of them destined to become in due course workers. He 

 estimated that in the course of a summer such a colony would 

 consume about a quart of "honey-dew". The aphids thrive so well 

 in the care of the ants that the association may be very harmful 

 to the plants on whose juices the aphids feed. 



FILTERABLE VIRUSES.— This is a general name for extremely 

 minute disease-causing microbes which can pass through a fine 

 porcelain filter, and without losing any of their virulence. The 

 first to be recognised (by Iwanowski in 1892) was the one that 

 causes "mosaic disease" in the tobacco plant. It was shown that 

 a healthy plant could be inoculated with filtered juice from diseased 

 leaves, and that the juice retained its power of infection for many 

 months — this clearly pointing to, if not proving, the presence of 

 a living microbe. 



But it was not till six years afterwards that the importance of 

 the new idea was recognised. In 1898 Loefifler and Frosch showed 

 that fluid taken from the blisters of cattle suffering from foot- 



