Biochemical characterization of defensive secretions in Heteropteryx dilatata: Compound identification and bioactivity assessment

Introduction

In contrast to the passive defense strategies of camouflage and catalepsy employed by most stick insects, several phasmid taxa have evolved active chemical defense mechanisms involving the production and deployment of glandular secretions [1,2] . Heteropteryx dilatata (Parkinson, 1798) is among the most conspicuously defended phasmid species, possessing prothoracic defense glands capable of spraying irritant secretions at perceived threats, complemented by powerful hind leg spines used in kicking defense [3] . Despite the ecological significance of chemical defense in this species, previous descriptions have been limited to behavioral observations and qualitative descriptions of secretion odor and color, without detailed chemical analysis [4,5] . Arthropod defensive secretions have proven to be rich sources of bioactive natural products with pharmacological potential, including antimicrobial peptides from beetles, terpenoid deterrents from termites, and cytotoxic compounds from millipedes [6,7] . The defensive chemistry of phasmid insects, while less extensively investigated than that of beetles or ants, has yielded several interesting compounds including actinidine, nepetalactone, and various monoterpenes [1,2] . Krejsa noted the defensive spraying behavior of H. dilatata but the chemical identity of the secretion components was not determined [5] . This research aimed to provide the first comprehensive biochemical characterization of H. dilatata defensive secretions through chromatographic separation, structural identification, and bioactivity screening.

Materials and Methods

Materials

Defensive secretions were collected from 40 adult H. dilatata specimens (20 males, 20 females) maintained in the arthropod vivarium of the Sao Paulo Foundation for Biochemical Research, Brazil. Specimens were healthy adults aged 4-8 months post-eclosion. Secretion collection was performed using a standardized mild mechanical stimulation protocol that elicited defensive spraying without causing injury. Secretions were collected directly into sterile glass capillary tubes, immediately transferred to amber vials, and stored at minus 80 degrees Celsius until analysis. Ethical approval was obtained from the institutional committee (SPFBR/CEUA/2023-078, approved 12 March 2023).

Methods

Chemical analysis employed dual chromatographic approaches. GC-MS analysis was performed on a Shimadzu GCMS-QP2020 NX system with an HP-5MS column (30 m x 0.25 mm x 0.25 micrometers) using helium carrier gas and a temperature gradient from 60 to 280 degrees Celsius. LC-MS/MS analysis was conducted on a Waters Xevo TQ-XS triple quadrupole system with an ACQUITY UPLC BEH C18 column (2.1 x 100 mm, 1.7 micrometers) for nonvolatile and thermally labile compounds. Compound identification was based on mass spectral library matching (NIST 20), retention index comparison, and co-injection with authentic standards where available. Bioactivity screening included antimicrobial disc diffusion assays against four bacterial strains and two fungal species, protease inhibition assays using trypsin and chymotrypsin substrates, and general cytotoxicity assessment using brine shrimp lethality assays. Temporal degradation kinetics were assessed by exposing secretion samples to ambient conditions (25 degrees Celsius, 60% relative humidity) and measuring residual activity at intervals from 0 to 72 hours.

Chemical Ecology of Phasmid Defense

The prothoracic defense glands of phasmid insects are modified exocrine glands that open via ducts on the anterolateral margins of the prothorax [1] . The glands exhibit substantial interspecific variation in size and secretory capacity, with species in the Heteropterygidae generally possessing larger glands relative to body size than most other phasmid families [2, 3]. The biochemical diversity of phasmid defensive secretions is increasingly recognized as an untapped resource for natural product chemistry.

Results

Discussion

The identification of 34 compounds across five chemical classes establishes H. dilatata as possessing one of the most biochemically complex defensive secretions documented among phasmid insects. The dominance of terpenes in the secretion profile is consistent with patterns observed in other chemically defended phasmids, including Anisomorpha buprestoides (which produces the monoterpene anisomorphal) and Peruphasma schultei (which produces a quinoline-based deterrent) [1,2] . The strong protease inhibition activity detected across multiple compound classes suggests a defensive mechanism targeting digestive enzymes of potential predators, which would impair the predator's ability to digest the prey item even if consumption occurs [6,7] . The significantly higher concentrations in female secretions correlate with the greater defensive investment expected from the larger sex, consistent with the size-defense allocation hypothesis observed across multiple arthropod taxa.

Conclusion

This research aimed to characterize the biochemical composition and bioactivity of defensive secretions in H. dilatata. The key findings include the identification of 34 compounds across five chemical classes, with terpenes predominating and protease inhibition representing the dominant bioactivity. The practical significance lies in the identification of several structurally unusual alkaloids that warrant further investigation as potential lead compounds for natural product drug discovery. Future directions should include structural elucidation of the unidentified alkaloids through nuclear magnetic resonance spectroscopy, assessment of dose-dependent deterrent effects on natural predators, and comparative analysis of secretion composition across populations from different geographic origins.

Acknowledgement

AcknowledgementsRHCO designed the research and supervised chemical analyses, CBFL performed GC-MS and LC-MS/MS analyses, LGAS conducted bioactivity screening assays, and JCMS assisted with secretion collection and sample preparation. This research was supported by the Sao Paulo Research Foundation (FAPESP). The Sao Paulo Foundation for Biochemical Research provided analytical instrumentation, reagents, and vivarium facilities.

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