June, 2008

Firefly Mimic Longhorn

We've had a delightful number of fireflies twinkling in our backyard over the past few weeks. While the slow-flying and soft-bodied fireflies appear to have few defenses against predators, they do have one very effective way to avoid being eaten: poisonous blood. The classic firefly color pattern of red head and black body is a standard way to tell enemies that they are not tasty morsels. Since many beetles already have the same shape as fireflies (which are also beetles), it has not been a great leap to adopt the same colors. Several other families of beetles, including soldier beetles and net-winged beetles, bear these same warning colors in order to fool predators.

The beetle featured this month is one of the best firefly mimics I've seen. The firefly mimic longhorn (Mecas rotundicollis) is actually in the family cerambycidae (longhorn beetles). It has the standard red head and black body, but also adds a detail that most other mimics lack: the light part on the end of the abdomen. This does not really light up like that of fireflies, but in daylight it looks surprisingly convincing. The light color does not even extend across the whole bottom of the insect, but just on the sides where it shows the most.

As with many beetles, these appear as adults for only a short period each year. This particular species makes the most of its disguise by showing up in our yard at the same time that firefly populations are at their height.