Spermatophylax

March 27, 2013 at 5:55 am (singing insects)
Tags: dusky-faced meadow katydid, Indiana Dunes National Lakeshore, Orchelimum campestre, spermatophore, spermatophylax

by Carl Strang

For such tiny creatures, insects have complex lives and biology. They have been shaped by natural selection in more ways than we know, but we know enough to be amazed. Recently I began expanding my readings on singing insects, and gained several insights on things I had noticed but hadn’t fully appreciated. Let’s begin with a photo from last summer.

This is the female dusky-faced meadow katydid I caught and photographed last summer at Indiana Dunes National Lakeshore.

Note the white gelatinous substance in her jaws. I cropped it out of the photos I shared in the blog then, but fortunately I was not so fastidious as to clean her up. This was a very important meal she was in the midst of consuming, according to Darryl T. Gwynne in his 2001 book, Katydids and Bush-crickets: Reproductive Behavior and Evolution of the Tettigoniidae (Cornell University Press). Another photo tells more of the story.

The female’s abdomen. I was focused on recording the shape of the ovipositor, but notice that more of the white substance is here, as well.

It turns out that this female had mated within the previous hour or two, her one and only time (this was surprising, but Gwynne says it is true of all Orchelimum meadow katydids). The male had inserted a spermatophore into her reproductive tract, with this gelatinous structure, called a spermatophylax, as an added extension. The female slowly eats the spermatophylax, which contains valuable nutrients, while fertilization is taking place. This is a significant investment by the male, about 10% of his body mass, and though he may mate again, it will take some time to build a new spermatophore.

Eventually she will eat the protein-rich sperm casing, as well, but by then her eggs will be fertilized. Gwynne studies the evolution of this system, and reviewed it across the worldwide spectrum of katydids in his book. There is some consensus among researchers that the spermatophylax originated as a distraction, preventing the female from immediately consuming the spermatophore and preventing fertilization (she could go on to mate as many times as she wished, building her nutrient reserves at the expense of the males whose sperm did not fertilize her eggs). The question remains, though, as to the degree to which the continued evolution of the spermatophore and its spermatophylax component improves the quality of the offspring by feeding the female. In the Orchelimum meadow katydids it seems that this issue is resolved. The female mates only once, and the male contributes a substantial nutrient gift that increases the size of her eggs. This is why it is really good that I released her without removing the spermatophylax material. It also may be why she was the one I caught, as her focus on her meal probably slowed her down.