The term "gynandromorph" literally means part female (gyn-) and part male (andro-). You might ask how such a thing can happen -- well, here's the answer. Stick with me here, as this requires some significant explanation!!
All sexually reproducing organisms begin existence as a single cell, a zygote, which is a fused sperm and egg cell. This cell then divides and divides, eventually making all the different cells in the body. During the process of cell division, some cells obviously become different from others (a process called differentiation) and ultimately a cell's development becomes determined at some point, that is, it enters a developmental pathway that determines what it will become (a muscle cell, a nerve cell, a cell lining the intestine, etc.).
In humans, the earliest cell divisions are indeterminate, which means that the developmental pathway is still flexible for all of the cells. An interesting side benefit of this is that, if one of these early cells happened to be destroyed, the end result is that it has virtually NO effect on development, since the other cells are still "flexible" in what they can become.
On the other hand, each cell division in insects, from the zygote on, is completely determinate, which means that decisions about what a cell will become are made with each division. The decisions that are made with the earliest cell divisions are as follows:
Just to follow up on what I said about humans, if an insect cell were to be damaged after the third division, say the lower back right cell, you would end up with seven-eighths of an insect (missing the back lower right part of the abdomen, for instance) if development could proceed fully.
Now, what does this have to do with gynandromorphs? I'll get there.
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!
So there you have it. Three of the four gynandromorphs I have pictured here are virtual bilateral gynandromoprhs, with the left side male and right side female in all cases (just coincidence), meaning the right side is lacking an X chromosome and that the non-disjunction of the X chromosome happened in the very first division of the zygote during development. Needless to say, this can result in a very striking looking individual!! Both the Tiger Swallowtail and the Diana Fritillary have two distinctly different looking sexes (called sexual dimorphism).
Tiger Swallowtail (Papilio glaucus) gynandromorph (left half male, right half female), © James
You can see the somewhat extended male clasper on the left side of the abdomen, and the male's yellow tegula
on the left side of the thorax. Collected at: Pigeon Mountain (west side), Walker Co., Georgia, April 16, 2004.
© Marc Perlman, 2004. This is an example of a mosaic
as mentioned above.
A nearly bilateral gynandromorph of Speyeria diana (a little
extra female on the upper left hindwing),
reared from female from Cooper's Creek WMA, Fannin/Union Cos., Georgia.
The left image is the upperside, the right is the underside.
© Irving Finkelstein. Note the line down the middle of the underside of the abdomen.
Malacosoma disstria gynandromorph (left half male, right half female), © James
You can even see a line directly down the middle of the body where the lighter, somewhat fluffier
scales of the male are visible on the left, and the darker scales of the female are visible on the right.
The female abdomen is also typically a bit longer than the male, which explains why the abdomen
is curled to the left (the longer female side is on the right).
Collected at: Micanopy, Alachua Co., Florida, April.