Well, we emailed the CO extension agency last year about this exact question, as I had 14 BSTs and only 3 of them were females. They were saying that it was determined when the eggs were fertilized. Since then, as I have researched and learned more, I can support this with what I've learned. It is determined as soon as the sperm combines with the egg cell by the chromosomes. I was just thinking one thing, but looking on the most helpful website I found, I found that the situation in butterflies is reversed from humans. Given that they already have it written out, I'll let them explain it. This site says: Sex is determined, in both humans and Lepidoptera, by sex chromosomes called the X & Y chromosomes. In humans, an individual that is male will have X & Y chromosomes, whereas a female will have two X chromosomes. In Lepidoptera, the situation is reversed -- XX is male and XY (or simply one X and no Y) is female. I should point out that it is the number of X's that is important. In Lepidoptera, an individual with just one X (and no Y) is going to be female. Any individual with two (or more) X's will be male. When cells divide, one complete copy of all the genetic information (DNA) is passed to both cells, or, in other words, before the division there are two complete copies of DNA in each of the chromosomes, which are attached to each other before division. These two copies of the DNA in each chromosome are then separated during cell division. Occasionally, the two copies of a particular chromosome do not detach from each other during cell division (called non-disjunction), and the end result is that one of the cells will end up missing an entire chromosome, which is typically lethal to the cell. The other cell (that gets the unseparated chromosome) may be unaffected, or normal, though problems can occur in this cell as well. However, if a non-disjunction occurs in an X chromosome in an individual that is XX (male in butterflies/moths), that would mean that one of the cells in this division would end up with one X chromosome, while the other would end up with two (or three) X's. In other words, one of the resulting cells (and all cells that came from that cell) would be female, while the other cell (and its descendents) would be male. So, if you have a non-disjunction in an X chromosome in an XX individual during the first division of the zygote, then you will end up with an individual that appears half male (on one side) and half female (on the other side). This is called a bilateral gynandromorph. The non-disjunction can occur during later divisions, however, giving you a smaller portion of the body/wings that looks like one sex and a larger portion that looks like another. It can even happen more than once during development, so that you end up with patches of female and male scattered around on the individual, resulting in what is called a mosaic (see second Tiger Swallowtail, below). VERY bizarre! Before I saw this site, I wasn't aware that there was such a thing as a mosaic individual. It sure is nice having the web at times... While thinking about this, I also realized another interesting fact. I realized that when a BF egg is first laid, it is 1 cell! After all, when a male and a female gamete combine, you end up with 1 cell that then divides into others. Since BF eggs are fertilized virtually as they are laid, that means that the BF egg you see at first is a single, extremely large cell! I mean, that is one giant cell (compared to most, at least)! -Jmcat

I always figured the best time for any moth or butterfly to emerge in spring was when their host plant has started to make new growth, Tiffy. Big moths are only supposed to live about a week, so if they emerge too early, even if they manage to find a partner and mate, what would the female lay her eggs on? She could lay them on bare branches and maybe the eggs could last long enough in cold weather, but wouldn't it make more sense for her to be able to lay her eggs on new growth, which is what they like? So, I put my cages on the front porch when new growth has begun - if I were you, whenever that happens where you are, that'd be the right time. Dan, I saw my first female spicebush swallowtail yesterday, and I think I saw a black female tiger today - I know it was a big black swallowtail, so maybe things are starting to look up. The cecropia and the luna males both left - maybe I'll get a female moth soon. Sherry

Late this afternoon I saw a pearl crescent flitting around in the cage! I intended to wait and release it tomorrow, since it probably didn't emerge until this afternoon, but it continued to flit around like it wanted to leave, so I got my camera, opened the cage, and it flew out before I could get a picture. It was a perfect and very vigorous little thing, looked like its mother, naturally. I don't know how to tell males from females, so I can't say which this was. Since one has emerged, the others will undoubtedly soon follow, and I'll try to get a picture of at least one of them. But woo hoo, I've successfully gone through the life cycle of PCs! Sherry

Sherry, the Polyphemus spins a white cocoon. Unlike the Luna, the Poly cocoon is quite dense. They spin a LOT of silk! Their cocoons look like tiny surgical casts. In fact, it is because of this, that the Poly's first cousin, native to China, Antheraea pernyi, was and may still be used for silk, just as the Bombyx is. The Cynthia moth is also used for silk. At one time, Cynthias were common in NE US, but have largely died off, chiefly due to habitat loss as well as parasitoids such as the tachinid fly. Susanlynne, one sureshot way to tell male from female Luna, at a glance, is color. The females are that characteristic green, while males are yellow-green. I could tell from my upstairs bedroom window, the Luna I saw, resting on a young hickory, was most definitely a male. No way could I see its body size or antennae. I hope this helps.