Head And Thoracic Morphology Of Surface-living And Cave-dwelling Ground Beetles

Cave biology(also known as “biospeleology”)has been an attractive subject since the first cave-dwelling animals were discovered in Slovenia.Subterranean animals are of increasing scientific interest for various reasons,not only displaying conspicuous morphological characteristics,but also physiological and behavioral modifications.Dark,moist and temperature-constant subterranean environments are an ideal natural laboratory to study important aspects of evolutionary theory,with ongoing debates among followers of competing hypotheses related to the evolution of cave organisms.Even though morphological adaptations of cave-dwelling organisms have fascinated researchers since a long time,very few detailed documentations of the anatomy of cave beetles are available.In this study we describe and illustrate external and internal cephalic and thoracic features of free-living and cave-dwelling trechinae carabid beetles(troglobite: Sinaphaenops wangorum Ueno et Ran 1998,troglophile: Trechiotes perroti Jeannel 1954,epigean: Bembidion sp.,Trechus sp.)using modern techniques,including SEM and micro-CT.Main results of our research are followings:(1)Head morphology: the gular ridges are fused with posterior tentorial arms in all of the examined cave-dwelling and surface-living beetles,wide anterior tentorial arms provide attachment area for M46 b,which has not been recorded in other adephagan beetles.Besides,the following features distinguish subterranean S.wangorum from other species: the elongation of the head capsule,posterior part of the gular ridge(behind tentorial bridge)is also elongated and doesn?t reach the postoccipital ridge,the antennae are markedly longer than others but no specific sensory organs were found.The compound eyes are completely reduced,and we haven?t found any remained structures on the ocular region;the optic lobes are completely missing.O n the other hand,several muscles of the troglobite beetles differ from the rest: M48(which is only composed of two thin fibers)and an additional dilator of the pharynx are present,and M29 b originates from the ventral head capsule anterad the gular apodeme,whereas it reach posterior parts of the capsule in other species.(2)Thoracic morphology: the hind wings of cave-dwelling S.wangorum and high elevation-living Trechus sp.are significantly reduced,just tiny non-flyable wings are visible.The comparison of thoracic skeletons among epigean and hypogean beetles showed strong modifications in the troglobites,such as bulged prothorax,reduced metanotum,phragma reduction and much bigger metafurca.Muscular systems of the four trechinae species(which live in deep cave,cave entrance,Tibetan highland,Tibetan lowland respectively)were compared,the prothoracic muscles are similar,but the meso-and metathoracic muscles appears different patterns.M5(related to the head movement)is only existed in S.wangorum,whereas M6 is observed in other three species.Unlike M6 which is attached to the gular ridge,M5 only reaches the postoccipital ridge.Strong muscles which related to flying and walking are observed in Bembidion sp.,but fewer muscles are found in T.perroti although it has complete wings.On the other hand,massive losses of muscles are existed in wing-reduced S.wangorum and Trechus sp..The comparison most of these losses are related to the flying and walking,this discovery may shed a new light on the adaptive process of hypogean organisms.Despite of conspicuous differences likely related with subterranean adaptations,the link of some cephalic and thoracic features to this specific habitat preference remains uncertain.In the future plans,more individuals and species should be involved,that might be helpful to a better understanding of the adaptive history of these hypogean creatures.