Insects Are Flying Shrimps, Myriapods Are Arthropod Snakes — Towards a New Synthesis
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Invertebrate Zoology, 2017, 14(2): 197–204 © INVERTEBRATE ZOOLOGY, 2017 Insects are flying shrimps, myriapods are arthropod snakes — towards a new synthesis D.E. Shcherbakov Borissiak Paleontological Institute RAS, Profsoyuznaya Str. 123, Moscow 117647, Russia. E-mail: [email protected] ABSTRACT: The first Atelocerata were machilid-like insects descended from Palaeocar- idacea (Malacostraca Syncarida) via neoteny, homeosis and gamoheterotopy. In these littoral hoppers, the five posterior segments of the malacostracan thorax were repatterned after abdominal ones to maintain the caridoid escape reaction at uncompensated gravity out of water. Their modified gonopods were shifted caudally and transferred from males to females to form an ovipositor. The vigorously flexing abdomen turned useless in cryptic habitats and was homeotically repatterned after the thorax in myriapods, which evolved as litter creepers from bristletail ancestors via entognathous hexapods. How to cite this article: Shcherbakov D.E. 2017. Insects are flying shrimps, myriapods are arthropod snakes — towards a new synthesis // Invert. Zool. Vol.14. No.2. P.197–204. doi: 10.15298/invertzool.14.2.15 KEY WORDS: Malacostraca, Insecta, Archaeognatha, Myriapoda, homeosis, thorax, abdomen. Насекомые как летучие креветки, многоножки как членистоногие змеи — к новому синтезу Д.Е. Щербаков Палеонтологический институт им. А.А. Борисяка РАН, ул. Профсоюзная 123, Москва 117647, Россия. E-mail: [email protected] РЕЗЮМЕ: Первые Atelocerata, насекомые напоминавшие махилид, произошли от Palaeocaridacea (Malacostraca Syncarida) путем неотении, гомеозиса и гамогетерото- пии. У этих прибрежных прыгунов пять последних сегментов рачьей груди были уподоблены брюшным, чтобы сохранить каридоидную реакцию бегства при неком- пенсированной гравитации вне воды. Модифицированные гоноподы самцов были сдвинуты к хвосту и перенесены на самок, превратившись в яйцеклад. Скрытоживу- щим формам не нужно брюшко, способное резко подгибаться, и оно было гомеоти- чески уподоблено грудному отделу у многоножек, приспособившихся к ползанию в подстилке и произошедших от махилид через энтогнатных гексапод. Как цитировать эту статью: Shcherbakov D.E. 2017. Insects are flying shrimps, myriapods are arthropod snakes — towards a new synthesis // Invert. Zool. Vol.14. No.2. P.197–204. doi: 10.15298/invertzool.14.2.15 КЛЮЧЕВЫЕ СЛОВА: Malacostraca, Insecta, Archaeognatha, Myriapoda, гомеозис, грудь, брюшко. 198 D.E. Shcherbakov Shrimps jump ashore possibly the Devonian (Devonocaris; Wells, 1957; Brooks, 1962). Many features of Palaeo- There are two alternative theories of insect caridacea were inherited by the Archaeognatha origin. The myriapod theory goes back to Brau- and other insects: the trunk of 14 segments plus er (1869): insects descended from myriapods, the telson; the carapace undeveloped; the 1st and the first insects were similar to Campodea. thoracomere free; the 1st and sometimes also 2– The crustacean theory was elaborated by Hans- 3rd thoracopods specialized as maxillipeds in en (1893): insects evolved from higher crusta- contrast to the 4–8th ones; thoracopods with a ceans (Malacostraca), and the first insects re- short exopod; pleopods simpler and smaller sembled Machilis. than thoracopods, with the endopod sometimes The crustacean theory, based on compara- reduced and in two (1–2nd) pairs produced to tive morphology and embryology (Crampton, form the male organ; uropods (their endopods 1918, 1938; Hansen, 1925, 1930; Sharov, 1966), homologous to insect cerci — Crampton, 1921) was substantiated by deep similarities in the and the telson sometimes spikelike; male go- structure of visual system (Nilsson, Osorio, nopores situated two segments more posterior 1998) and brain (Strausfeld, 2009), neurogene- than in females; and the embryonic dorsal organ sis (Whitington, Bacon, 1998), and expression of the same structure and position (Sharov, patterns of segmentation genes (Patel, 1994). 1966). Molecular phylogenetics also supports the crus- The insects most closely resembling mala- tacean-insect relationships. In at least some costracans are jumping bristletails (Archaeog- molecular trees, the sister group of insects is the natha, since the Triassic — Montagna et al., Malacostraca (Garcia-Machado et al., 1999; 2017) and related Monura (Carboniferous–Tri- Wilson et al., 2000). assic, possibly the Devonian — Labandeira et As Crampton (1922) has put it, “Further- al., 1988). Jumping bristletails represent the more, there is no possibility of being deceived ancestral, crustaceoid type better than any other by “convergent” development in this instance, living hexapod (Crampton, 1938). They retain since the remarkable resemblance, both ana- numerous malacostracan features: tagmosis with tomical and embryological, present in so wide a rather smooth transitions between the head, series of structures from such different parts of thorax and abdomen; the tail fan (cerci, i.e. the body, and extending even to the minutest limbs of the 11th abdominal segment, and the details, in [crustaceans and insects], can be paracercus); a full set of abdominal limbs (free- explained only as the result of consanguinity. ly projecting, styli-bearing coxites participating Since convergent development is supposedly in locomotion); exopods (coxal and abdominal the result of the effects of similar environmental styli); abdominal endopods (eversible vesicles); conditions, it is difficult to believe that the leg bases with coxo-trochanteral joints close to environment of a marine crustacean (or even a the midline; paranotal lobes concealing limb littoral one) can have enough in common with bases; compound eyes structurally similar to the environment of a terrestrial insect (in some those of Crustacea; the naupliar eye transformed cases mountain-dwelling ones) to produce the into ocelli; huge 1st antennae with an annulate astoundingly close similarity one finds in the flagellum; the supramandibular suture; limb- minutest structural details in the two groups of like maxillary palps; well-developed paragnaths arthropods!” (hypopharynx); and the 2nd maxillae (fused The crustaceans most similar to insects are basally to form the labium) similar to the 1st the Eumalacostraca (known since the Devo- maxillae. The rolling machilid mandibles with a nian), particularly the Syncarida (Tillyard, 1930), separate incisor, elongated mola and postero- and, of these latter, the Palaeocaridacea, known dorsal adductor muscle are sub-ectognathous, from the Carboniferous and Early Permian with their posterior articulation hidden under (Schram, 1984, 1986; Perrier et al., 2006), and the paranotal fold of the mandibular segment Flying shrimps, arthropod snakes 199 (Manton, 1964) and functionally dicondylic, [Syncarid] to become a Machilid, as far as with incipient anterior articulation, i.e. like in segmentation is concerned, is for the last five Zygentoma, so there is no reason to abandon the thoracic segments to change their function and concept of the Thysanura s.l. (Kluge, 1996). become abdominal, with consequent reduction Insects are neotenic in retaining such embry- of their appendages to vestiges!” I suggest that onic syncarid characters as sessile eyes and five posterior segments of the malacostracan uniramous 1st antennae and uropods (=cerci). thorax were homeotically repatterned after ab- These transformations correlate with terrestri- dominal ones to maintain the escape reaction in alization, as well as the loss of a swimming larva open terrestrial habitats, so the insect thorax (nauplius) with suppression of its natatory limbs corresponds to the maxillipedal part of the ma- (2nd antennae and mandibular telopodites = lacostracan thorax. It was five segments that palps), and acquisition of tracheae (Boudreaux, underwent transformation because hexapody is 1979). Probable rudiments of the 2nd antenna optimal for a faster gait. Furthermore, the set of are found in modern machilids, campodeids and modified gonopods (and the expression domain grasshoppers (Crampton, 1932). of the underlying homeotic gene Abdominal-B; Because the segment number is irrelevant Averof, Akam, 1995) was shifted caudally and for determining homology of tagmata (Minelli, transferred from males to females to form an Peruffo, 1991), three principal tagmata — head, ovipositor, allowing insertion of eggs into sub- thorax and abdomen — can be traced through- strate crevices, which is adaptive in open-living out the Arthropoda. The sets of homeotic genes terrestrial forms (Fig. 1). controlling this tagmosis are basically similar in Such transformation of a syncarid into a all arthropods (Carroll, 1995). Each segment of machilid took place through heterochrony the arthropod body is capable of producing a (neoteny), heterotopy (homeosis) and gamohet- fully developed basically biramous limb or, in erotopy, i.e. shift of characters from one sex to insects, both leg and wing, the latter being another (Meyen, 1988). Unlike heterochrony, homologous to the distal epipodite of the crus- heterotopies produce no parallelisms between tacean limb (Averof, Cohen, 1997). Therefore, ontogeny and phylogeny: instead of resembling a ground plan segment is thoracic, whereas in an ancestor at a different developmental stage, the limbless, e.g. abdominal, segments limb a novelty created by heterotopy will differ from development is suppressed (Carroll et al., 1995). any stage of the ancestral ontogeny (Zelditch, The arthropod abdomen likely appeared as Fink, 1996). an integral locomotory organ, initially