The black-orange-black color pattern has been well studied in the orders Lepidoptera (butterflies, moths) and Coleoptera (beetles), but a new study is the first to fully examine Hymenoptera (wasps, bees, ants, and sawflies) to document the “BOB” color pattern within that taxonomic order, finding it present in 23 families. (Photo credit: Rebeca Mora)

By Andrew Porterfield

It’s no surprise that a class as diverse as Insecta should show an enormous diversity in body coloration. Camouflage, mate attraction, mimicry, repelling predators, and warning signs all are factors in the evolution of insect colors.

Andrew Porterfield

But one distinct pattern, black-orange-black, appears frequently in Lepidoptera (butterflies, moths) and Coleoptera (beetles) and even has been detected in wasps, bees, ants, and sawflies of Hymenoptera. The coloration is mostly seen as a black (or dark brown) head, orange mesosoma, and black metasoma. This pattern has not been extensively studied, either for its prevalence or the evolutionary reasons for its existence.

One of the first studies to determine the extent of the black-orange-black (BOB) pattern was recently conducted by Rebeca Mora, researcher in biology at the University of Costa Rica, and published last week in the Journal of Insect Science. By examining specimens of Hymenoptera from Costa Rica and Canada, Mora and her team found BOB coloration patterns in 23 Hymenoptera families and found variations in BOB patterns depending on the size of the insect, altitude, and taxa.

Mora and her colleagues looked at nearly 500,000 specimens in two museums in Costa Rica and 783,000 specimens at the Canadian National Collection of Insects, Arachnids and Nematodes in Ottawa, Ontario, Canada. The specimens represented collections of insects from around the world.

A study of the black-orange-black color pattern in the order Hymenoptera found 66 variations on the pattern, including a dozen common variations illustrated here. (Mts = metasoma, Prp = propodeum, Scu = scutellum, Mes = mesoscutum, H = head.) (Image originally published in Mora and Hanson 2019, Journal of Insect Science)

The researchers found 66 orange and black patterns, representing a striking degree of variation. There also was a distinct tendency of the BOB pattern to appear in species dwelling at altitudes below 2,000 meters. Other black-orange patterns were also seen, including a black head and mesosoma (and orange metasoma) everywhere except Indonesia and Malaysia; a black head and mesosoma with a mixed orange and black metasoma everywhere except tropical Africa and Austral-asia, and a black head with orange mesosoma and metasoma (except a black tip) in all regions studied.

Smaller insects also were more likely to exhibit the BOB pattern, especially in hymenopterans with a body length between three and 10 millimeters. In addition, the BOB pattern was seen in all regions, and not just the tropics, as previous studies have suggested. On the other hand, most of the unique patterns seen were in three areas: tropical Africa, Indonesia/Malaysia, and Austral-asia.

“We were surprised by how widespread this color pattern is among hymenopterans from all biogeographic regions,” says Mora. “We knew this color pattern is common in scelionids but did not realize how common it is in several other groups, such as evaniids. We were also surprised by how uncommon it is in chalcidoidea, nor did we expect so many variations on the basic BOB color pattern.”

A mixed sample of scelionid wasps illustrates the common occurrence of a black-orange-black color pattern in Hymenoptera. A new study finds the color pattern present in at least 23 families within the taxonomic order. (Photo credit: Rebeca Mora)

It is not exactly clear why Hymenoptera (or other orders) exhibit the BOB pattern, though Mora suggests that the most likely reason is “aposematism,” or warning coloration. This same type of pattern has been studied in other insect orders and has been shown to be effective at warning off predators. Some species showing BOB might also be attempting to mimic ants.

“Compared with butterflies and beetles, there are relatively few studies of color patterns in Hymenoptera, and what few studies exist are mostly restricted to larger-sized species that sting,” says Mora. “The BOB color pattern is most common in smaller wasps, especially scelionid wasps. These small wasps are difficult to observe in the field and they are usually collected by trapping or sweeping with a net.”

The study underscored the need for more research on the taxonomy of hymenopterans, since most small species in the order have yet to receive scientific names and identity. “Only then can we determine what proportion of species have the BOB pattern,” says Mora.

Future research on the BOB pattern should come from several areas, she says, including studies on the physical and chemical properties of the insect cuticle, pigment chemistry, and biological functions of the color patterns.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.