Actias luna Cocoon Data

Our luna moth (Actias luna) larvae have all finished spinning cocoons.

---

Since all the larvae have spun up, we went ahead and collected all the cocoons from the tubs and ripped off all the leaves. There are just 19 cocoons total which is very few for how many we started with, but we're not too worried though as it is not the most valuable species.

While rearing the larvae, we had put them in an opaque tub starting in the fourth instar which blocked out light when closed, allowing us to limit the light to only a few hours a day. Thus, we expected that all the cocoons would diapause and be made of tough, brown silk than the thin beige of the normal, non-diapausing ones. However, after inspecting all the cocoons, we saw that there was a very large variation in silk color, with some being extremely dark brown, others being almost white, and many being an intermediate orange-brown. The dark brown and probably also orange-brown ones should be in diapause, but we can't be sure about the beige ones. They look just like the spring cocoons that did not diapause, but perhaps the silk just hasn't hardened and tanned yet.

Some other interesting things about this rearing's cocoons are that one of the cocoons is extremely large, thin, and baggy, rather than the normal round and compact. It sort of resembles the baggy form of Hyalophora cecropia cocoons, though this form is not supposed to exist in luna. Perhaps something about the way the available attachment points were arranged while the prepupa was spinning caused it to produce such an odd cocoon. Other unusual cocoons were two pairs of fused cocoons due to the larvae spinning right next to each other. We were able to split them apart without damaging them with some difficulty. 

Anyway, on to the mass data. After collecting the cocoons, we weighed and recorded their masses and calculated some summary statistics, shown below: 
​
Summary Statistics for Actias luna Cocoon Mass (Summer 2017):
Mean     Std Dev    Median     Min     Max      N
2.750      0.541          2.68            1.98      3.86       19

For comparison, we pulled up the spring stats as well:

 Summary Statistics for Actias luna Cocoon Mass (Spring 2017):
Mean     Std Dev    Median     Min     Max      N
3.525      0.411            3.4              3.0        4.2        12​

What's the first thing we noticed? That's right - a ridiculously lower summer cocoon mass compared to spring. Sample size is small, but the difference is still quite extreme. A two-sample t-test for the difference of means between spring and summer cocoon mass also supports that the difference is statistically significant (t = 4.2357; df = 29; p=0.0002).

Right now the first explanation to come to mind is simply the severe overcrowding the summer larvae experienced in later instars. But really, it wasn't too much worse by the fifth instar compared to in the spring rearing. Food quality should have been about the same, so the only other difference was the temperature and humidity. The spring larvae were incubated at constant 27°C and high humidity during the first three to four instars while the summer larvae were at room temperature their entire lives. However, this seems to be a contradictory factor, since from what we've seen, the high constant temperature of the incubator tends to speed larval growth drastically at the cost of size. Plus, by the fifth instar - the critical growing period, both spring and summer larvae were at room temperature. The only other factor that could have played a significant role in size reduction was inbreeding. The summer larvae were inbred offspring of the outbred spring larvae. It is hard to say how exactly the recessive gene stacking affects the larvae, but from what we suspect and what some other hobbyists claim based on observation is that inbreeding tends to reduces size. We only inbred for one generation, but perhaps that was enough to cause this extreme drop in size.

---

Authors
Alan Liang

---