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Balance, Hearing, and More: Why Some Wasps Have Fat Lower Legs

Gasteruption longipleurale

Wasps in the family Gasteruptiidae, such as Gasteruption longipleurale, are notable for their unique features such as an elongated neck and enlarged tibial sections of their hind legs. A new study published in Insect Systematics and Diversity finds these wasps’ enlarged tibiae turn out to be filled with insect fat body, and they may play key roles in flight dynamics, detecting vibrations from prey, and even detoxification. (Image originally published in Mikó et al 2019, Insect Systematics and Diversity)

A notable feature on some wasp species is their long rear legs that dangle as they fly—indeed, it’s hard to miss if one buzzes nearby. In one family of wasps, Gasteruptiidae, which comprises about 500 species, those dangly legs are also enlarged in the tibial section, giving them the look of bodybuilders who overworked their calf muscles. Entomologists have long wondered why exactly this odd feature evolved in these wasps, and a new study offers our best look yet at what those large legs are for.

To find out, a team of researchers led by István Mikó, Ph.D., at Penn State University dissected leg sections from some common gasteruptiid wasps and examined them through a range of analyses:

  • multiple types of microscopy, to obtain various optical views of the legs and the tissues and cells within
  • two forms of spectral analysis, to reveal the chemical signatures within the leg tissues
  • genetic sequencing, to detect what genes are highly expressed in the leg tissues.

The researchers also observed the wasps’ flight behavior, both under normal conditions and with their rear tibiae removed, to examine the legs’ role in balance and maneuvering.

What they found seemed to support previous hypotheses—that the legs are involved in flight dynamics as well as sensing delicate vibrations from the wasps’ prey—but the study also revealed some surprises, first and foremost being that the gasteruptiid wasps are filled with fat body, an insect organ almost exclusively associated with the abdomen. Their results were published this month in Insect Systematics and Diversity.

As a gasteruptiid wasp seeks out larvae of solitary and soil-nesting bees to parasitize, she engages in a vertical and horizontal swaying behavior in flight. Research on this behavior and the physical characteristics of gasteruptiids’ enlarged lower legs suggests that fat body in the leg adjacent to the wasp’s subgenual organ (used to detect minute vibrations) is used to amplify vibrational signals, and the leg swaying “could be a way to tune into the source of these vibrational signals through the air.” (Video originally published supplementary to Mikó et al 2019, Insect Systematics and Diversity)

“Besides the classical fat body functions—i.e. detoxification, fat metabolism—it is possible that this structure plays an important role in transmission of vibrational signals,” says Mikó, now collection manager at the University of New Hampshire Collection of insects and other arthropods. “This study demonstrates that we should be more careful with generalizations in biology and that basic morphological observations still play an important role in 21st century entomological sciences.”

Projections of chordotonal organ

Researchers studying the enlarged lower leg section of gasteruptiid wasps used confocal laser scanning microscopy to create a three-dimensional view of the leg interior. This animated view of the basal portion of the gasteruptiid tibia (tibial wall green) shows the subgenual organ (orange) and the wall of the dilated trachea (purple/blue).” (Image credit: István Mikó, Ph.D.)

Gasteruptiid wasps are parasitoids of solitary and soil-nesting bees; a female gasteruptiid seeks out its host’s larvae in their nest, where she then deposits her egg. Like many insects, gasteruptiids’ legs feature a subgenual organ, known to be used in detecting minute vibrations. The fat body in the gasteruptiids’ enlarged hind tibiae is located adjacent to the subgenual organ, suggesting the fat body serves to amplify those vibrational signals. Mikó and colleagues note that the wasps’ vertical and horizontal swaying of their legs in flight near their hosts’ nests “could be a way to tune into the source of these vibrational signals through the air.”

The presence of fat body in the wasps’ legs raises questions on its own, but a closer look at the cells in the tibial tissue and genes expressed therein raises even more. Within the tibial fat body, the researchers found what appear to be oenocytes, cells unique to insects responsible for lipid processing and detoxification and (like fat body) usually found in the abdomen. Meanwhile, highly expressed genes in wasps tibiae were ones linked to detoxification, lipid synthesis and transport, and mechanoreceptors—collectively, “characteristics expected more of insect abdomen than a normal hind tibia,” the team notes in their report.

“Although we can better understand these wasps with this study and better explain their unique behaviors and morphologies, perhaps more exciting is the general implications for the evolution of form,” says Heather Hines, Ph.D., assistant professor of biology and entomology at Penn State and senior author on the study. “There is a lot to discover about basic physiological and morphological functions of insects. … Learning the function of a structure is not necessarily straightforward, and not many people bother to take a closer look, but now more than ever we have the tools at our disposal to provide a holistic perspective and get closer to answers.”

Indeed, the study was a collaboration of 12 scientists spanning four universities on three continents to bring together a diversity of analytical techniques.

“I am an insect morphologist who aims to discover and describe unknown anatomical systems primary using microscopic techniques. It was eye opening to see how much more we can tell about anatomical structures using tools from genomics and material sciences,” Mikó says. “I hope that similar collaborations will be more and more common in the era of convergence research.”

gasteruptiid tibia 3D model screenshot

Mikó and colleagues created an interactive 3D model of a gasteruptiid leg. Download the 3D PDF [160 MB]. (To enable interactive features, open the file with Adobe Acrobat Reader.)

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