Color Variation in P. nana and S. mexicana

Over the years, we noticed some interesting differences in color morphs in Mediterranean Katydids (Phaneroptera nana) and Mexican Bush Katydids (Scuderria mexicana) we have found.

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Color variation is extremely common among insects just like humans exhibit an endless array of unique skin tones. In cryptic species, the most common major color groups are green and brown, for obvious reasons. Through the years we have encountered both of these color forms in both Mediterranean (Phaneroptera nana) and Mexican Bush (Scudderia mexicana) katydids though it is not quite the same between the species.

In P. nana, we have found the brown morph to be relatively more common, perhaps making about about 10% of the population based on our sample size of ~40 katydids collected from the same area (passion flower and bramble at El Cerrito's Ohlone Greenway at Lincoln Ave., an area in direct sunlight with both green and brown backgrounds). On the other hand, the brown form is much more rare in S. mexicana, having only found 3-4 of them since we first encountered them in 2008.

However, the age of the katydid must be accounted for. We have observed that all of the brown morph S. mexicana katydids we have encountered have always been very young nymphs, never larger than the fourth instar. When we have reared these katydids, they have consistently turned green by the time they have reached adulthood and therefore some of the larger green morph katydids we have found may have been brown in younger instars. It is also important to note that in both species, the first and second instars are very dark in color. At the same time the change to green could actually be due to various differences in the captive environment as compared to the environment in which they have been found such as light intensity, humidity, or background color (all common stimuli for color change in insects). However, this is somewhat unlikely since our captive enclosure was located outside in a clear tank with a screening lid and live plants and should have had a similar amount of sunlight exposure and humidity as other outdoor environments. In addition, background color probably does not play a role in determining color because we have found brown morphs on green backgrounds and vice versa and as well as both color forms on the same color background at the same time in both species. We have also never observed a green morph katydid turn into a brown one.

On the other hand, we have not witnessed as dramatic of a change in color between nymphs and adults in P. nana. Although we have yet to see the adult form of the brown morph nymphs we are currently keeping, over the instars they have not yet completely shown any signs of turning green (the change should be gradual over several instars). Our largest brown morphs are already in 6th instar, two instars greater than we have ever found brown morph S. mexicana nymphs and one instar away from adult. However, when we did raise a brown morph nymph to adulthood in 2014 in the same outdoor enclosure as the color-changing S. mexicana, it resulted in a green-brown intermediate which does suggest that it would have turned completely green had it had the chance to molt a few more times. This year, we are rearing them indoors, so if our current brown morphs do eventually result in completely brown adults perhaps it will suggest that the color change was actually related in some way to light intensity or humidity. 

In addition to the brown and green color forms in S. mexicana and P. nana, there are also variations in secondary colors such as in the red stripes. In S. mexicana, red stripes and coloration are always present but vary greatly in intensity with some being almost nonexistent (creating an almost completely green look) while in others it is very pronounced (creating a dark purplish, almost black undertone). There is a correlation between this intensity of red and overall darkness in shade of the individual; for example, darker green individuals tend to have more and darker red (most noticeable on the wings and genitals in adults). This may very well be correlated with light intensity because we have often found that individuals that have already been reared indoors for a large portion of the lives tend to be darker than the ones we encounter in sunny locations. On the other hand, the red abdominal stripe are uncommon in P. nana during younger instars and virtually all of our younger nymphs are a mostly solid colored. However, it seems that only green individuals will have the red stripe in later instars, though brown morphs with the red stripe could still exist since we have a small sample size of brown morphs. We have yet to make any correlations between this and light intensity. At the same time, the red in both species is most definitely controlled in part by genetics, though we also have not tested this. 

In conclusion, the color differences within these two species of katydids is somewhat mysterious, to say the least. Unlike many other insects, it has been very hard for us to pinpoint the exact mechanism controlling these phenotypes but perhaps we will find the answer soon now that we are rearing more katydids than we ever had in the past this year. 

The top row shows the green and brown forms of the Mexican Bush Katydid; the bottom shows the same for the Mediterranean Katydid.

- Brian