1865.] 



201 



ceeded in forcing two out of twenty-seven tessellaris to feed upon syc- 

 amore for about forty days, till they finally spun up ; yet to the very 

 last they retained all their own distinctive characters, and showed not 

 the slightest disposition to assume those of the other species. What is 

 very remarkable, out of forty-three Harrisli that I fed this year upon 

 oak, not a single one lived to spin up; but so long as they remained 

 alive in the breeding-cage, which was on the average about five days, 

 the only approximation that they made towards tessellu7'iswsiS, that their 

 hairs (not their pencils) generally became of a dirtier white. 



In order to judge of the eft'ects of food upon these two insects, I have 

 this year tried the following experiments under precisely similar con- 

 ditions, and with every possible precaution to guard against error, l.s-^ 

 I fed upon oak a lot of feasdlarls found upon Oak, in Breeding-cage 

 No. 3. 'Ind. I fed upon Oak a lot of tessellaris found partly on Thorn, 

 partly on Basswood, and partly on Hickory, in Breeding-cage No. 4. 

 Zrd. I fed upon kSycamore a lot of tessellaris found partly on Oak and 

 partly on Basswood, in Breeding-cage No. 5. 4:th. I fed upon Oak 

 two distinct lots of //cow's a found upon Sycamore, in Breeding-cages 

 Nos. 6 and 7. x\n accurate record was kept of the phenomena pre- 

 sented by each lot, which, for the sake of brevity and for the conveni- 

 ence of comparisons, has been reduced to the following tabular abstract. 

 About three-fourths of the larvEe in each lot were quite young, ranging 

 from .30 to .50 inch in length, and scarcely any were admitted that 

 appeared to be more than half grown. 



*I give here and elsewhere what is, strictly speaking, the average number, 

 and not the medium number. The two things are often confounded together, 

 but are quite different. For example, if one larva dies in 4 days and three die 

 in 8 days, the average number of days is seven, but the medium number is six. 

 I suspect that certain writers, who profess to give the average dimensions of 

 insects, give, in reality, nothing but the medium dimensions. To calculate the 

 average dimensions of forty or fifty specimens, requires that every one of the 

 forty or fifty should be measured. To calculate the medium dimensions, all that 

 is necessary is to measure the smallest and the largest. 



