# The Thoracic Anatomy of Two Flightless Chrysolina Species (Coleoptera: Chrysomelidae)

## TL;DR

This study compares the thoracic anatomy of two flightless beetles to understand how flight loss affects their body structures and possible compensatory adaptations.

## Contribution

The study provides the first detailed thoracic anatomical comparison of two flightless Chrysolina beetles using multiple morphological methods.

## Key findings

- Both species lack flight muscles and cervical sclerites but differ in elytral base structure and setae.
- C. sulcicollis has fewer elytra-controlling muscles and more body-stabilizing muscles, possibly for water conservation.
- The absence of cervical sclerites and presence of specific muscles may enhance head mobility in flightless beetles.

## Abstract

Flight loss has independently occurred in almost all winged insect groups. Studying the thoracic structures of flightless insects and comparing them to their flying relatives can help identify traits linked to flight. In beetles, especially the diverse Chrysomelidae family, most research has focused on taxonomy and distribution, with little attention to thoracic anatomy. This study compares the thoracic structures of two flightless Chrysolina species—C. sulcicollis from a desert region and C. virgata from a temperate region—using various morphological methods. Both species show reduced flight muscles and lack of cervical sclerites, but differ in elytral base structure, mesal suture shape, and epipleuron setae. C. sulcicollis has fewer elytra-controlling muscles and more muscles for body stabilization, likely due to water-saving needs. Compared to other flight-capable chrysomelid beetles, the two flightless species have similar thoracic skeletons, except for the missing flight-related muscles. The absence of cervical sclerites, combined with the presence of certain neck-related muscles, might help flightless beetles move their heads more freely, potentially compensating for their inability to fly. Also, the higher number of tergo-pleural muscles in the mesothorax of C. virgata could suggest that its elytra have a special function. Better understanding their evolutionary paths requires more data on environmental factors and the trade-offs linked to flight loss.

Flight loss has independently evolved across nearly all winged insect orders. Comparing the thoracic structures of flightless insects with those of their flight-capable relatives can reveal key characteristics linked with flight. Although flight loss has been widely studied in beetles, exploration of this phenomenon has been limited to taxonomic and geographic distribution studies in the species-rich family Chrysomelidae, with little analysis of thoracic anatomical structures. This study employs a suite of morphological techniques to examine the thoracic structures of two flightless beetle species Chrysolina: sulcicollis and Chrysolina virgata, originating from desert and temperate regions, respectively. A comparison between the two flightless species reveals that C. sulcicollis has fewer tergo-pleural muscles involved in elytral movement likely to save water, but more muscles that contribute to stabilizing larger body structures. Meanwhile, differences are also observed in the elytral base, the anterior corner of the mesal suture, and the setae on the meso-inner region of the epipleuron. Compared to other flight-capable chrysomelid beetles, apart from the absence of flight-related muscles, the two flightless beetles exhibit similar thoracic skeletal structures. The absence of lateral cervical sclerites, along with the presence of muscles Idvm4, 5 and Itpm5, could enhance head mobility as a compensatory adaptation doe the loss of flight capability. Additionally, the greater number of tergo-pleural muscles in the mesothorax of C. virgata could suggest that its elytra serve a specialized function. Compared to other flightless beetles, aside from the similarly reduced flight muscles, these two species have relatively intact thoracic skeletons. Further data on habitat, functional compensation and other related factors are needed to compare their evolutionary processes with those of other flightless beetles.