Multi-scale relationships between numbers and size in the evolution of arthropod body features

An anatomical and ultrastructural study of the eye of the luminescent millipede Paraspirobolus lucifugus (Gervais 1836) (Diplopoda, Spirobolida, Spiroboleliidae)

The two forward-looking eyes and their ultrastructural organization of an 18 mm long adult bioluminescent female millipede (Paraspirobolus lucifugus) were investigated by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Each eye contained approximately 23 ommatidia with 50-60 μm wide and 80 um thick corneal lenses that contained calcium and silicon and proximally ended in truncated flat surfaces of around 20 μm in diameter. A maximally 28 μm thick and 25 μm long rhabdom, made up of at least 12-14 retinula cells and a 4 μm thick sleeve of screening pigment granules in a light-adapted position was present. Compared with the eyes of non-luminescent julid millipede species, those of P. lucifugus share their basic anatomy, but also exhibit features like the wide possible binocular frontal visual overlap, somewhat narrower interommatidial angles combined with relatively larger rhabdoms, which suggests that P. lucifugus has more efficient eyes and makes greater use of its photoreceptors. P. lucifugus is negatively phototactic and strictly nocturnal and its activity rhythm is apparently governed by a circadian clock.

Effects of thermal and oxygen conditions during development on cell size in the common rough woodlice Porcellio scaber

During development, cells may adjust their size to balance between the tissue metabolic demand and the oxygen and resource supply: Small cells may effectively absorb oxygen and nutrients, but the relatively large area of the plasma membrane requires costly maintenance. Consequently, warm and hypoxic environments should favor ectotherms with small cells to meet increased metabolic demand by oxygen supply. To test these predictions, we compared cell size (hindgut epithelium, hepatopancreas B cells, ommatidia) in common rough woodlice (Porcellio scaber) that were developed under four developmental conditions designated by two temperatures (15 or 22°C) and two air O2 concentrations (10% or 22%). To test whether small-cell woodlice cope better under increased metabolic demand, the CO2 production of each woodlouse was measured under cold, normoxic conditions and under warm, hypoxic conditions, and the magnitude of metabolic increase (MMI) was calculated. Cell sizes were highly intercorrelated, indicative of organism-wide mechanisms of cell cycle control. Cell size differences among woodlice were largely linked with body size changes (larger cells in larger woodlice) and to a lesser degree with oxygen conditions (development of smaller cells under hypoxia), but not with temperature. Developmental conditions did not affect MMI, and contrary to predictions, large woodlice with large cells showed higher MMI than small woodlice with small cells. We also observed complex patterns of sexual difference in the size of hepatopancreatic cells and the size and number of ommatidia, which are indicative of sex differences in reproductive biology. We conclude that existing theories about the adaptiveness of cell size do not satisfactorily explain the patterns in cell size and metabolic performance observed here in P. scaber. Thus, future studies addressing physiological effects of cell size variance should simultaneously consider different organismal elements that can be involved in sustaining the metabolic demands of tissue, such as the characteristics of gas-exchange organs and O2-binding proteins.

No limits: Breaking constraints in insect miniaturization

Small arthropods are not simply scaled-down versions of their larger closest relatives, as changes in morphology and functional characters are largely governed by scaling laws. These same scaling laws set strict limits to size change toward smaller sizes. The evolution of extreme miniaturized forms involves the breaking of these constraints, by means of design innovations that allow evolutionary change to evade the limits posed by scaling laws. Here we review several cases studies in insects and other arthropods that illustrate this evolutionary path. We examine morphologies commonly recurring in miniaturized forms but not exclusive to them, morphologies exclusive to miniaturized forms and novel functional solutions supported by unconventional morphologies. We also discuss miniaturization and its evolvability taking into consideration arthropod postembryonic development and modular body organization. The modification of features commonly supposed not to change appears as a recurring pattern in arthropod miniaturization.

The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size

Stable attachment of a leading-edge vortex (LEV) plays a key role in generating the high lift on rotating wings with a central body. The central body size can affect the LEV structure broadly in two ways. First, an overall change in the size changes the Reynolds number, which is known to have an influence on the LEV structure. Second, it may affect the Coriolis acceleration acting across the wing, depending on the wing-offset from the axis of rotation. To investigate this, the effects of Reynolds number and the wing-offset are independently studied for a rotating wing. The three-dimensional LEV structure is mapped using a scanning particle image velocimetry technique. The rapid acquisition of images and their correlation are carefully validated. The results presented in this paper show that the LEV structure changes mainly with the Reynolds number. The LEV-split is found to be only minimally affected by changing the central body radius in the range of small offsets, which interestingly includes the range for most insects. However, beyond this small offset range, the LEV-split is found to change dramatically.

Cell size versus body size in geophilomorph centipedes

Variation in animal body size is the result of a complex interplay between variation in cell number and cell size, but the latter has seldom been considered in wide-ranging comparative studies, although distinct patterns of variation have been described in the evolution of different lineages. We investigated the correlation between epidermal cell size and body size in a sample of 29 geophilomorph centipede species, representative of a wide range of body sizes, from 6 mm dwarf species to gigantic species more than 200 mm long, exploiting the marks of epidermal cells on the overlying cuticle in the form of micro-sculptures called scutes. We found conspicuous and significant variation in average scute area, both between suprageneric taxa and between genera, while the within-species range of variation is comparatively small. This supports the view that the average epidermal cell size is to some extent taxon specific. However, regression analyses show that neither body size nor the number of leg-bearing segments explain this variation, which suggests that cell size is not an usual target of change for body size evolution in this group of arthropods, although there is evidence of its correlation with other morphological variables, like cuticle thickness. Scute sizes of miniaturized geophilomorph species are well within the range of the lineage to which the species belong, suggesting recent evolutionary transitions to smaller body size.

Nymphalid eyespots are co-opted to novel wing locations following a similar pattern in independent lineages

BACKGROUND

Variation in the number of repeated traits, or serial homologs, has contributed greatly to animal body plan diversity. Eyespot color patterns of nymphalid butterflies, like arthropod and vertebrate limbs, are an example of serial homologs. These eyespot color patterns originated in a small number of wing sectors on the ventral hindwing surface and later appeared in novel wing sectors, novel wings, and novel wing surfaces. However, the details of how eyespots were co-opted to these novel wing locations are currently unknown.

RESULTS

We used a large data matrix of eyespot/presence absence data, previously assembled from photographs of contemporary species, to perform a phylogenetic investigation of eyespot origins in nine independent nymphalid lineages. To determine how the eyespot gene regulatory network acquired novel positional information, we used phylogenetic correlation analyses to test for non-independence in the origination of eyespots. We found consistent patterns of eyespot gene network redeployment in the nine lineages, where eyespots first redeployed from the ventral hindwing to the ventral forewing, then to new sectors within the ventral wing surface, and finally to the dorsal wing surface. Eyespots that appeared in novel wing sectors modified the positional information of their serial homolog ancestors in one of two ways: by changing the wing or surface identity while retaining sector identity, or by changing the sector identity while retaining wing and surface identity.

CONCLUSIONS

Eyespot redeployment to novel sectors, wings, and surfaces happened multiple times in different nymphalid subfamilies following a similar pattern. This indicates that parallel mutations altering expression of the eyespot gene regulatory network led to its co-option to novel wing locations over time.

Developmental mechanisms of body size and wing-body scaling in insects

The developmental mechanisms that control body size and the relative sizes of body parts are today best understood in insects. Size is controlled by the mechanisms that cause growth to stop when a size characteristic of the species has been achieved. This requires the mechanisms to assess size and respond by stopping the process that controls growth. Growth is controlled by two hormones, insulin and ecdysone, that act synergistically by controlling cell growth and cell division. Ecdysone has two distinct functions: At low concentration it controls growth, and at high levels it causes molting and tissue differentiation. Growth is stopped by the pulse of ecdysone that initiates the metamorphic molt. Body size is sensed by either stretch receptors or oxygen restriction, depending on the species, which stimulate the high level of ecdysone secretion that induces a molt. Wing growth occurs mostly after the body has stopped growing. Wing size is adjusted to body size by variation in both the duration and level of ecdysone secretion.

ChiloKey, an interactive identification tool for the geophilomorph centipedes of Europe (Chilopoda, Geophilomorpha)

ChiloKey is a matrix-based, interactive key to all 179 species of Geophilomorpha (Chilopoda) recorded from Europe, including species of uncertain identity and those whose morphology is known partially only. The key is intended to assist in identification of subadult and adult specimens, by means of microscopy and simple dissection techniques whenever necessary. The key is freely available through the web at: http://www.biologia.unipd.it/chilokey/ and at http://www.interactive-keys.eu/chilokey/.