307) NOCTUID LARVAE— RIPLEY 65 



able whether or not this correction, brought about in consequence of the 

 longer duration of the last larval stadium, where this condition is found, 

 is sufficiently extensive to render significant a comparison of the slopes 

 of the last two units without further correction. Possibly these two units 

 may remain equal as they stand in Plate I, the long last stadium having 

 taken care of the correction, which would otherwise have to be introduced 

 by increasing the length of the next to last unit. Where we find a long first 

 stadium we should perhaps increase the length of the first unit even more 

 than otherwise. It is well to recall at this point, however, that the length 

 of a stadium may not be an index to the amount of postembryonic develop- 

 ment undergone during it, since, as has been previously suggested, the 

 longer stadium may be correlated with a slower development. If this be 

 true the long first stadium requires no greater correction than the shorter 

 one. 



When we compare different parts of the same curve, then, with refer- 

 ence to slope, it must be remembered that the units should not be of equal 

 length, as they stand in Plate I, but that each should be somewhat longer 

 than the one which follows it. The possibility that the last two units may 

 need Uttle or no alteration in order to represent the true condition should 

 also be considered. Furthermore, the first unit may require greater length- 

 ening than otherwise for species with a long first stadium, such as Dip- 

 terygia scabriuscula. The general eflect of this correction is to make the 

 primary curving downward on the left of the chart very gradual and to 

 accentuate the secondary curving upward on the right. Upon applying 

 this correction mentally to Plate I, we note that this secondary shortening 

 of the epicranial stem has proceeded with much greater rapidity than its 

 primary lengthening. It becomes evident, moreover, that this primary 

 process has generally accelerated with the passing of time, altho the curves 

 seem to indicate the opposite condition before the necessary correction is 

 applied. Straight lines, where they occur on the uncorrected chart, do not 

 indicate a constant rate of evolution, but an acceleration. The secondary 

 shortening process has also progressed at an increasing rate, the accelera- 

 tion being much greater than in the lengthening process. 



The two species of the genus Phytometra examined present a distinct 

 type of curve in which the epicranial index remains unchanged thruout 

 the first two periods, followed by the usual accelerated lengthening. The 

 epicranial stem of the last instar of these larvae is as short as that of the 

 average species whose curve turns upward. This condition is evidently due 

 to the fact that in this genus the primary lengthening has been delayed 

 until the third period, so that the epicranial stem has not been evolving in 

 this direction for a sufficiently long time to enable it to attain the length 

 common to species in which this suture has not undergone secondary 

 reduction. 



