INSECT LARVAE AS TRACEMAKERS: COMPOUND STRUCTURES AND ONTOGENETIC VARIATIONS

Several families of Diptera are known to be active burrowers. Fly larvae belonging to the genus Symplecta (family Limoniidae, subfamily Chioneinae) were observed in a sandy substrate of an orange grove in Lepe, Huelva, SW Spain, with more than 20 specimens studied. Complete forms display three distinctive segments: (1) a proximal part characterized by a winding furrow trail (unbranched, unlined, medial furrow; (2) an intermediate part consisting of burrow segments arranged in a zigzag pattern; and (3) a distal part formed by a simple, asymmetric U-shaped burrow. The most complex morphology is present in the intermediate zigzag burrow system which may vary from irregularly sinuous and curved in its most proximal part to spiraled (both dextrally and sinistrally) in its distal part. If this biogenic structure were found in the fossil record, it would be ascribed to different ichnotaxa depending on the part preserved. In fact, this structure does not reflect a single behavioral pattern, but a compound trace that records multiple behaviors. The winding furrow shows affinities with Helminthopsis, the zigzag burrows with Treptichnus, and the U-shaped burrows with Arenicolites. Furthermore, the spiral portion shows some similarities with Spirophycus and Spirodesmos. Interestingly, formation of these compound traces is connected with different ontogenetic stages of the producer. The winding and zigzag segments are produced by chioneinae larvae for feeding purposes, representing pascichnia to fodinichnia strategies, while the U-shaped burrow is excavated during the pupal stage for protection, recording a domichnion that the organism inhabited until adulthood. Symplecta has a worldwide distribution and inhabits riparian and lacustrine littoral zones. In the fossil record, this compound trace potentially may occur in lake margin and floodplain deposits, including proximal overbank areas and ponds, forming part of the Scoyenia or, less likely, the Mermia ichnofacies.