Swollen Tarsi in Jurassic Scorpionflies: Fossils Reveal Sexual Selection in Ancient Insects

More than 160 million years ago, during the Middle Jurassic, a group of scorpionflies (order Mecoptera) became preserved within the fine lacustrine sediments of the Jiulongshan Formation in what is now northeastern China. These fossils, discovered in deposits belonging to the Yanliao Biota, have provided an unexpected window into the reproductive biology of ancient insects. Among eighty-seven fossil specimens examined, researchers discovered a peculiar morphological feature: the first tarsal segment of the hind legs was dramatically swollen in certain individuals, a condition never seen in typical insect legs. Intriguingly, this swollen structure appears exclusively in male specimens. This seemingly minor anatomical detail has allowed paleontologists to infer aspects of mating behavior, sexual dimorphism, and even broader ecological dynamics of Jurassic insect communities.

The eighty-seven specimens studied include eighty-six belonging to the extinct family Orthophlebiidae and a single representative of the rarer family Holcorpidae, represented by the newly described species Conicholcorpa longa. In this species, the sixth through eighth abdominal segments are greatly elongated, forming a conspicuous posterior structure typical of males within that lineage. Even more striking, however, is the pronounced swelling of the first tarsal segment on the hind legs of males in both families. Female specimens show no trace of this modification.

To determine whether this swelling carried biological significance, researchers measured the length and width of the tarsal segments in each specimen and calculated their aspect ratios. This allowed them to quantify the degree of swelling and compare it statistically across specimens. The results clearly demonstrated that swollen segments were substantially broader than ordinary tarsi, confirming that the feature represents a real morphological trait rather than deformation or preservation artifacts.

Fossil of Conicholcorpa longa. c–f Close-up views of the first tarsal segment of the hind leg（Image source：Zhang YJ et al. (2021)， CC BY 4.0 ）

The degree of swelling varies markedly among species. For example, Orthophlebia elenae shows only a modest enlargement, whereas Mesopanorpa luanpingensis displays a much more pronounced expansion. In Orthophlebia extensa, the swelling becomes extreme, producing a strongly bulbous structure. The swollen tarsi often appear irregularly shaped and distorted compared with normal tarsal segments, creating a conspicuous morphological contrast. Such variation suggests that the swelling represents a distinct anatomical type rather than a random anomaly. Moreover, differences in swelling patterns among species indicate that this trait may hold taxonomic and phylogenetic significance.

Comparison of the first tarsal segment of the hind leg in different species; a, b Orthophlebia extensa (non-swollen, female); c, d Orthophlebia elenae (slightly swollen); e, f Juraphlebia eugeniae; g, j Mesopanorpa luanpingensis (heavily swollen); h, i Orthophlebia extensa (extremely swollen)（Image source：Zhang YJ et al. (2021)， CC BY 4.0 ）

The fact that swollen tarsi occur exclusively in males strongly suggests a role connected with reproduction. Modern scorpionflies are well known for their unusual courtship behavior. In living members of the families Panorpidae and Bittacidae, males frequently present females with so-called nuptial gifts before mating. These gifts may consist of captured prey or edible secretions produced by the male's salivary glands. By offering such a gift, the male increases the likelihood that the female will accept copulation and may also prolong the duration of mating, thereby improving the chances that his sperm successfully fertilizes the eggs.

Orthophlebiidae, the dominant family represented among these Jurassic fossils, shares evolutionary ancestry with modern scorpionflies. Because of this relationship, researchers propose that Jurassic scorpionflies may also have practiced a similar gift-giving strategy during courtship. If males carried prey items to present to females, the enlarged tarsal segments could have played a visual role in enhancing the appearance of the offering.

One intriguing hypothesis suggests that the swollen first tarsal segment functioned as a form of "gift camouflage." When a male grasped prey with his hind legs, the exaggerated tarsal structure may have made the item appear larger or more impressive to a potential mate. In effect, the morphology could have helped transform a modest food item into something visually more substantial, increasing the male's attractiveness during courtship displays.

Comparable deceptive strategies occur in several modern insects. Some dance flies (family Empididae), for instance, produce empty silk balloons or wrap inedible items such as plant seeds to imitate prey gifts when courting females. These visual tricks allow males to gain mating opportunities while investing fewer resources in actual food items. Jurassic scorpionflies may have employed a simpler but conceptually similar strategy, using exaggerated anatomy to enhance the perceived value of their offerings.

The fossils also reveal intriguing details about the physical structure of the swollen tarsi. The surface coloration and fragility of the cuticle appear to correlate with the degree of swelling. Segments that are only slightly enlarged tend to display smooth, light-colored cuticle. In contrast, strongly swollen segments often show darker irregular patterns, and in extreme cases the cuticle appears fractured. This pattern suggests that the swelling stretched the exoskeleton, thinning the cuticle and making it more susceptible to breakage. Despite these dramatic structural changes, statistical analyses indicate that the swelling is unrelated to the lengths of the hind tibia or the wings, reinforcing the interpretation that it represents a specialized morphological adaptation rather than a general scaling effect of body size.

The evolutionary relationships between Orthophlebiidae and Holcorpidae have long been debated. Various studies have proposed different placements for these groups within the broader Mecoptera lineage. The shared presence of swollen first tarsal segments in both families suggests that they may form a closely related lineage. If this interpretation is correct, the swollen tarsus could represent a derived trait that evolved in their common ancestor rather than an independent development within a single species.

Reconstruction of orthophlebiid scorpionflies（Image source：Zhang YJ et al. (2021)， CC BY 4.0 ）

Taken together, these fossils suggest that Jurassic scorpionflies inhabited a world shaped by intense sexual competition. Males appear to have evolved both elongated abdominal segments and exaggerated tarsal structures to enhance their chances of attracting females. Such features likely functioned as display structures within complex courtship interactions.

Sexual selection can drive the evolution of highly exaggerated anatomical traits, but these adaptations often carry costs. Structures that become too large or conspicuous may require greater metabolic investment to produce and maintain. They may also increase the risk of detection by predators. Over long evolutionary timescales, such trade-offs can influence the rise and fall of entire lineages.

The swollen tarsi of Jurassic scorpionflies therefore offer more than an anatomical curiosity. They reveal that elaborate mating strategies and sexual display behaviors were already shaping insect evolution deep in the Mesozoic. Through the delicate preservation of these fossils, the silent lake sediments of the Jiulongshan Formation have preserved not only the bodies of ancient insects, but also clues to the complex behavioral interactions that once unfolded in Jurassic ecosystems.

Author: Shui-Ye You

Reference:

Zhang YJ et al. (2021). Jurassic scorpionflies (Mecoptera) with swollen first metatarsal segments suggesting sexual dimorphism. BMC Ecology and Evolution.

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