Smerinthus opthalmica Summer 2017 Pupa Mass Data

7/23/2017

We collected the pupa mass data from the Smerinthus ophthalmica pupae of our summer 2017 rearing.

Today concludes our mass rearing of Smerinthus ophthalmica this summer as all larvae have now pupated. yes, it's quite terrifying just how many pupae we have now if you just look at the pictures. We'll be in for a real disaster if they don't diapause and all start eclosing right before we head off to Ithaca. For now, we've transferred them all to a large tub and spread them out evenly and will be checking for signs of development when we expect them to eclose (if not diapausing, probably in a week).

So, on to the mass data. Over the past two weeks, we've been actively recording the masses of every viable pupae the day it pupated and have just entered it all in a spreadsheet today. The total count for our rearing in 275 pupae, 141 male and 134 female (though just today we discovered two were dead, but since we already recorded their masses they are included in the data). Below are summary statistics and a stacked histogram showing the frequency distribution of pupa mass by sex, as well as singular histograms for each sex.

Summary Statistics for Smerinthus ophthalmica Pupa Mass (Summer 2017)
Mean Std Dev Median Min Max N
Male 1.966666667 0.2316606714 2.0 1.2 2.6 141
Female 2.28880597 0.3147168313 2.3 1.5 3.2 134
All 2.123636364 0.3186213667 2.1 1.2 3.2 275

Though a statistician could definitely do quite a bit of analysis here, the main things to notice are that the distribution of the combined female and male data set is nearly unimodal and symmetric with a few outliers on each end. Oddly, however, we should be expecting a bimodal distribution here because through performing a two-sample t-test, we found that the difference of means between female and male pupa was extremely statistically significant (t = 9.7001; df = 273; p<0.0001).

When looking at the histograms for the individual sexes, the distribution for both are more or less unimodal or symmetric, especially the male's, so nothing unusual there. If it is like the case with human height, then perhaps despite that the difference between masses of the individual sexes is significant, the variation in the distribution of both are so great that the combined graph cannot illustrate the difference, hence its lack of bimodality. But then again, we're not statisticians....

Comparison with our spring rearing:
Another point of interest in collecting all this mass data is comparing to mass data from previous rearings. Below are the summary statistics and histogram for our spring 2017 rearing.

Summary Statistics for Smerinthus ophthalmica Pupa Mass (Spring 2017)
Mean Std Dev Median Min Max N
Male 2.481818182 0.2358736179 2.4 2.1 2.9 11
Female 3.02 0.2936362073 3.05 2.5 3.4 10
All 2.738095238 0.377460184 2.7 2.1 3.4 21

Though the sample size was much smaller, it is quite obvious that the spring masses are higher on average. The difference of means between spring and summer pupae is 0.614±0.144 (C=0.95), for male pupae is 0.515±0.144 (C=0.95), and for female pupae is 0.731±0.203 (C=0.95). A two-sample t-test also showed that the difference of means between the two rearings was extremely statistically significant (t = 8.4037, df = 294, p<0.0001).

The incredible size difference between our two rearings is certainly due largely to the severe overcrowding and poorer leaf quality (leaves were more mature later in the season and keeping cuttings fresh with water tubes became impractical) in our summer rearing which greatly reduced the average mass of the pupae. Anyone who has reared larvae in mass quantities has experienced this.

Other factors could have been genetics, which seems to play a surprisingly large role in the size of these and similar moths, as we have seen quite a large size difference between different lineages of larvae in the past (no statistical evidence, though). The parents of the larvae in the summer rearing were those from the spring rearing, but the majority of the parents were some of the smallest of the brood (we tried to pair large ones, but the timing of the eclosions made this difficult). The only decently large pairing we managed to obtain was the last one, which produced the youngest larvae of our summer rearing, which, we indeed noticed seemed larger than the older ones (the 3.1 and 3.2 females and 2.5 and 2.6 males were among of the last to pupate). Thus, the smaller sized pupae seen in the summer rearing could be partly explained by genetics due to the bottlenecking of "large" genes from our spring rearing.

And finally, the last little thing to discuss for our mass data: the exuviae. Over the weeks, we collected every single exuviae shed by the prepupae upon their pupation out of curiosity to see just how much mass loss from prepupa to pupa is accounted for by the exuviae. Because a single piece is too light to register on the scale, we consolidated all the exuviae in a single petri dish and tared the collective mass which read 14.7 g. The total mass of the 275 pupae we recorded is 584 g. This means that the prepupa loses about 0.0535 g, or 0.0246% of their mass upon pupation! Now that's quite a thick cuticle they got there!

Authors
Alan Liang

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