Early leaf miners and the ground plan of the lepidopteran larval trunk: Caterpillar morphology of the basal mothsHeterobathmia,Eriocrania, andAcanthopteroctetes

Abstract

The larval trunk morphology including chaetotaxy, locomotory structures, and trunk musculature of Heterobathmia pseuderiocrania, Eriocrania cicatricella, and Acanthopteroctetes unifascia is described using conventional light, polarization, and scanning electron microscopy. The ground plan morphology of the lepidopteran larva and neolepidopteran caterpillar is discussed in light of the life history succession from free soil dwelling organism to endophagous and finally to a primarily free living, angiosperm associated organism. I suggest that the larval morphology is argued to be strongly influenced by the shift in number of surfaces present in the larval environment. Especially the environment of the endophagous species, where the upper surface of the leaf mine is linked to the presence of dorsal locomotory structures such as the retractable calli and dorsal friction patches is proposed to have had a significant impact on the morphology and locomotory mechnism of the lepidopteran caterpillar. The chaetotaxy of the lepidopteran ground plan is found to be simple, consisting only of primary and secondary tactile setae and segmental proprioceptors. The presumption of Gerasimov ([1935] Zool Anz 112:177-194) that MXD1 of the prothorax is a shifted mesothoracic MD setae is supported. I suggest that the serial arrangement of the proprioceptors MD1, present on all trunk segments except the prothorax, and a trisetous MV group on all the thoracic segments is part of the lepidopteran larval ground plan. The absence of apodeme structures associated with trunk musculature in the nonglossatans suggests that this is an autapomorphic character of the Lepidoptera and it is further found to have been influential in the evolution of the typical caterpillar trunk. The attachments of the thoracic muscles directly to the trunk integument, suggest that the apodemal structures ancestral to the Amphiesmenoptera have been reduced in the Lepidoptera. Within the non-Neolepidoptera, the lifehistory shift may have resulted in reduction of the dorsal locomotory structures, such as calli. The abdominal musculature and structural similarities further suggest that the ventral calli are structural predecessors to the crotchet bearing proleg of the "typical caterpillar."

Chambered cuticle, pellicles, strange sensilla, and extraordinary muscle arrangements: A study of the micropterigid larval trunk (Lepidoptera: Micropterigidae)

“…The micropterigid arrangement of the dorsal longitudinal muscles into two widely separated bands is markedly different from the single‐level configuration thought to be ancestral for lepidopteran larvae (Dupont, ). This arrangement and the absence of lateral musculature between the two longitudinal bands indicate that the hump‐backed, slug‐like trunk shape of micropterigid larvae evolved primarily trough augmented growth of the dorsolateral integument.…”

“…Both species bear four setae on the integument below the leg sclerotizations; the most anterior of these is different from the other three. These four setae are considered to be homologues of the coxal setae of the heterobathmids as well as to the four setae found on what is interpreted as vestigial coxal areas of eriocraniids and agathiphagids (Dupont, ).…”

“…The absence of internal sclerotisations and apodemes in the larval trunk is an apomorphy ascribed to the lepidopteran ground plan (Dupont, ). The larval musculature of Micropterigidae however, bears little resemblance to that of any of the other nonneolepidopterans.…”

“…Muscles vep 4 of the mesothorax and metathorax extend into the segment in front, allowing the thoracic segments to be contracted on the ventral side. Ventral thoracic muscles that extend across intersegmental folds are not present in Eriocrania and Heterobathmia (Dupont, ) and are also unrecorded from the thorax of hepialids (Birket‐Smith, ) and examined Mecoptera ( Panorpa : Barlet, ) and Trichoptera ( Hydropsyche : Barlet, ; Limnephilus : Tindall, ); hence the vep 4 configuration is potentially a micropterigid autapomorphy. The absence in Sabatinca of muscles overlapping the segment boundary between the metathorax and abdominal segment I is shared with the other basal Lepidoptera described including hepialids (Birket‐Smith, ) and is considered the lepidopteran groundplan conditian (Dupont, ).…”

“…Ventral thoracic muscles that extend across intersegmental folds are not present in Eriocrania and Heterobathmia (Dupont, ) and are also unrecorded from the thorax of hepialids (Birket‐Smith, ) and examined Mecoptera ( Panorpa : Barlet, ) and Trichoptera ( Hydropsyche : Barlet, ; Limnephilus : Tindall, ); hence the vep 4 configuration is potentially a micropterigid autapomorphy. The absence in Sabatinca of muscles overlapping the segment boundary between the metathorax and abdominal segment I is shared with the other basal Lepidoptera described including hepialids (Birket‐Smith, ) and is considered the lepidopteran groundplan conditian (Dupont, ). Indeed, in Sabatinca a prominent difference between the metathorax and abdominal segment I on one hand and abdominal segmens II–VII on the other is the absence in the former of counterparts of the segment‐border‐crossing muscles vea 1 and vem 1.…”