This article reports on the chemical analysis of molecular profiles of tarsal secretions of the desert locust
Schistocerca gregaria (Forsskål, 1775) by gas chromatography hyphenated with quadrupol mass
spectrometry (GC–MS) as well as
1
H-nuclear magnetic resonance (
1
H NMR) spectroscopy. Special focus
of this study was to elaborate on sampling methods which enable selective microscale extraction of
insect secretions in a spatially controlled manner, in particular tarsal adhesive secretions and secretions
located on cuticle surfaces at the tibia. Various solvent sampling procedures and contact solid-phase
microextraction (SPME) methods were compared in terms of comprehensiveness and extraction
efficiencies as measured by signal intensities in GC–MS. Solvent sampling with water as extraction
solvent gave access to the elucidation of chemical profiles of polar compound classes such as amino acids
and carbohydrates, but is extremely tedious. Contact SPME on the other hand can be regarded as a
simplified and more elegant alternative, in particular for the lipophilic compound fraction. Many
proteinogenic amino acids and ornithine as well as carbohydrate monomers arabinose, xylose, glucose,
and galactose were detected in tarsal secretions after acid hydrolysis of aqueous extracts. Qualitatively
similar but quantitatively significantly different molecular profiles were found for the lipid fraction
which contained mainly n-alkanes and internally branched monomethyl-, dimethyl-, and trimethylalkanes in the C23–C49 range as well as long chain fatty acids and aldehydes. Especially, hydrocarbons
with >C40 carbon numbers have previously been rarely reported for insect secretions. The results suggest
that the investigated insect secretions are complex emulsions which allow the attachment of tarsi on various otherwise incompatible materials of smooth and rough surfaces. The solid consistence of the
established alkanes at ambient temperatures might contribute to a semi-solid consistence of the
adhesive, amalgamating partly opposing functions such as slip resistance, tarsal release, desiccation
resistance, and mechanical compliance. The methods developed can be extended to other similar
applications of studying compositions of insect secretions of other species.