Robber flies, also known as assassin flies, are predatory insects found worldwide. While their appearance might raise concerns, understanding their behavior can alleviate fears about their danger to humans and pets. This article clarifies what robber flies are and how to manage encounters.
Identifying Robber Flies
Robber flies belong to the family Asilidae, a diverse group with over 7,000 species globally and more than 1,000 in North America. These insects typically exhibit elongated bodies, often with a somewhat humpbacked appearance, and possess bristly legs designed for capturing prey. Their size can vary significantly, with most adults ranging from 0.5 to 0.75 inches, though some species can reach up to 1.25 to 2 inches in length.
A distinctive feature of robber flies is their head, which has a noticeable depression between their two large compound eyes. Many species also have a “bearded” face, characterized by long hairs around their mouthparts. They are frequently observed in open, sunny areas such as grasslands, fields, gardens, and near ponds and streams, where they perch conspicuously to spot their insect prey.
Assessing the Threat to Humans and Animals
Robber flies are aggressive predators of other insects, but not towards humans or pets. They only bite defensively if mishandled or provoked. A bite can be painful, similar to a bee sting, involving the injection of digestive enzymes and venom.
This venom primarily immobilizes insect prey. While the bite causes localized pain, mild swelling, and irritation, it is not medically significant or dangerous to humans or pets. Robber flies do not transmit diseases or feed on blood. They play a beneficial role in ecosystems by preying on various insects, including pests.
Managing Encounters with Robber Flies
When encountering a robber fly, leave it undisturbed. They typically fly away if not provoked and are not inclined to approach or attack. If one lands on you, gently brush it away rather than swatting or grabbing it, as this can trigger a defensive bite.
Should a bite occur, simple first aid can alleviate discomfort. Clean the area with soap and water to prevent infection. Apply a cold compress or ice pack for 10 to 20 minutes to reduce pain and swelling. Over-the-counter anti-itch creams, such as calamine lotion or hydrocortisone, or a baking soda paste, may also provide relief. Medical attention is rarely necessary unless signs of an allergic reaction, such as difficulty breathing or widespread swelling, develop, or if the bite shows signs of infection like increasing redness or pus.