Evaluating old truths: Final adult size in holometabolous insects is set by the end of larval development

Ultrastructural Changes in Final Instar Larvae of Papilio polytes (Lepidoptera: Papilionidae) Lead to Differences in Epidermal Spreading of Water and Adjuvants

Papilio polytes is a cosmopolitan Lepidoptera species of controversial use and management. It remained unclear how its epidermal ultrastructure changes during development and how this affects its wetting properties in relation to water and pesticide adjuvants. In this study, the epidermis of P. polytes was systematically examined at the important feeding stage (from 3rd to 5th instar). Its ultrastructure was quantitatively observed by scanning electron microscopy. Its wetting properties towards the three main types of adjuvants and water were evaluated by contact angle. The chemical functional group differences between different instars and different adjuvant treatments were analyzed by mid-infrared spectroscopy. The correlation between the ultrastructural deformation and variations in wetting properties was verified by simulation tests. It was found that the complication of the epidermal structure was the leading factor for the significant increase in hydrophobicity during development. Cationic adjuvants had the best infiltrating effect on complex epidermal structures and organosilicon adjuvants had the best infiltrating effect on simple epidermal structures. The results provide data for biomimetic design for different wetting properties and suggest the feasibility and advantages of selecting pesticide adjuvants based on developmental changes in the structural characteristics of the insect epidermis.

Untangling plastic responses to combined thermal and dietary stress in insects

Animals are exposed to changes in their environmental conditions daily. Such changes will become increasingly more erratic and unpredictable with ongoing climate change. Responses to changing environments are influenced by the genetic architecture of the traits under selection and modified by a range of physiological, developmental, and behavioural changes resulting from phenotypic plasticity. Furthermore, the interactions between multiple environmental stressors to which organisms are exposed can generate unexpected phenotypic responses. Understanding how genetic and plastic variation contributes to the response to combined environmental stress will be key to predicting how animals will cope with climate change and ultimately will define their ability to persist. Here, we review the approaches used to explore how animals respond to combined stressors, specifically nutrition and temperature, the physiological mechanisms that underlie such plastic responses, and how genetic variation alters this plasticity.

Food hardness preference reveals multisensory contributions of fly larval gustatory organs in behaviour and physiology

Food presents a multisensory experience, with visual, taste, and olfactory cues being important in allowing an animal to determine the safety and nutritional value of a given substance. Texture, however, remains a surprisingly unexplored aspect, despite providing key information about the state of the food through properties such as hardness, liquidity, and granularity. Food perception is achieved by specialised sensory neurons, which themselves are defined by the receptor genes they express. While it was assumed that sensory neurons respond to one or few closely related stimuli, more recent findings challenge this notion and support evidence that certain sensory neurons are more broadly tuned. In the Drosophila taste system, gustatory neurons respond to cues of opposing hedonic valence or to olfactory cues. Here, we identified that larvae ingest and navigate towards specific food substrate hardnesses and probed the role of gustatory organs in this behaviour. By developing a genetic tool targeting specifically gustatory organs, we show that these organs are major contributors for evaluation of food hardness and ingestion decision-making. We find that ablation of gustatory organs not only results in loss of chemosensation, but also navigation and ingestion preference to varied substrate hardnesses. Furthermore, we show that certain neurons in the primary taste organ exhibit varied and concurrent physiological responses to mechanical and multimodal stimulation. We show that individual neurons house independent mechanisms for multiple sensory modalities, challenging assumptions about capabilities of sensory neurons. We propose that further investigations, across the animal kingdom, may reveal higher sensory complexity than currently anticipated.

Applying nutritional ecology to optimize diets of crickets raised for food and feed

Increasing yield is a primary goal of mass insect rearing for food and feed, and diet impacts insect life-history traits that affect yield, such as survival, development time and body size. However, experiments rarely test the nutritional requirements of insects from hatch to adulthood, and so little is known about how the full developmental macronutrient intake impacts the survival, growth and adult body size of mass-reared insects. Here, we applied the nutritional geometry framework and reared individual tropical house crickets (Gryllodes sigillatus) from hatch to adulthood on a wide range of protein : carbohydrate diets. We measured weekly food consumption, survival, development time to adulthood and adult body size and mass, and calculated a yield metric to extrapolate our individual-level results and predict how diet influences yield at the mass-rearing level. Yield was maximized on a 3P : 1C diet, as crickets fed this diet were most likely to develop into adults and grew maximum mass and body size. When provided with a choice between diets, crickets selected a relatively balanced 1.05P : 1C diet throughout development, but males consumed 17% more protein than females. Our results represent a crucial first step towards determining the optimal standard feed formulation required to maximize cricket farming yield.

Older mothers produce smaller eggs without compromising offspring quality: a study of a thelytokous mite predator (Acari: Phytoseiidae)

Negative relationships between the parental age and offspring life history traits have been widely observed across diverse animal taxa. However, there is a lack of studies examining the influence of parental age on offspring performance using mites, particularly phytoseiid predators as subjects. This study explored the influence of maternal age on offspring life history traits in Amblyseius herbicolus (Chant) (Acari: Mesostigmata), a phytoseiid predatory mite reproducing through thelytokous parthenogenesis. We hypothesised that increased maternal age negatively impacts offspring traits, including developmental duration, body size, fecundity and lifespan. Amblyseius herbicolus was reared under controlled laboratory conditions, and the life history traits of offspring from mothers of varying ages were analysed using linear mixed-effect models. Our results showed that the increase in maternal age significantly reduced individual egg volume, but did not significantly affect offspring developmental duration, body size, fecundity or lifespan. These findings indicate that while older A. herbicolus females produced smaller eggs, the subsequent performance (i.e. body size, fecundity and lifespan) of offspring remained largely unaffected, suggesting possible compensatory mechanisms in the offspring or alternative maternal provisioning strategies. The results of this study offer useful insights into the reproductive strategies of phytoseiid predators and asexually reproducing species, enhancing our understanding of how maternal age affects offspring fitness. Further studies can examine how offspring of A. herbicolus from mothers of different ages perform under adverse environmental conditions.

Synchronizing Drosophila larvae with the salivary gland reporter Sgs3-GFP for discovery of phenotypes in the late third instar stage

The larval stage of the Drosophila melanogaster life cycle is characterized by rapid growth and nutrient storage that occur over three instar stages separated by molts. In the third instar, the steroid hormone ecdysone drives key developmental processes and behaviors that occur in a temporally-controlled sequence and prepare the animal to undergo metamorphosis. Accurately staging Drosophila larvae within the final third instar is critical due to the rapid developmental progress at this stage, but it is challenging because the rate of development varies widely across a population of animals even if eggs are laid within a short period of time. Moreover, many methods to stage third instar larvae are cumbersome, and inherent variability in the rate of development confounds some of these approaches. Here we demonstrate the usefulness of the Sgs3-GFP transgene, a fusion of the Salivary gland secretion 3 (Sgs3) and GFP proteins, for staging third instar larvae. Sgs3-GFP is expressed in the salivary glands in an ecdysone-dependent manner from the midpoint of the third instar, and its expression pattern changes reproducibly as larvae progress through the third instar. We show that Sgs3-GFP can easily be incorporated into experiments, that it allows collection of developmentally-equivalent individuals from a mixed population of larvae, and that its use enables precise assessment of changing levels of hormones, metabolites, and gene expression during the second half of the third instar.

Brain size scaling through development in the whitelined sphinx moth (Hyles lineata) shows mass and cell number comparable to flies, bees, and wasps

Factors regulating larval growth and determinants of adult body size are described for several holometabolous insects, but less is known about brain size scaling through development. Here we use the isotropic fractionation ("brain soup") method to estimate the number of brain cells and cell density for the whitelined sphinx moth (Lepidoptera: Hyles lineata) from the first instar through the adult stage. We measure mass and brain cell number and find that, during the larval stages, body mass shows an exponential relationship with head width, while the total number of brain cells increases asymptotically. Larval brain cell number increases by a factor of ten from nearly 8000 in the first instar to over 80,000 in the fifth instar. Brain cell number increases by another factor of 10 during metamorphosis, with the adult brain containing more than 900,000 cells. This is similar to increases during development in the vinegar fly (Drosophila melanogaster) and the black soldier fly (Hermetia illucens). The adult brain falls slightly below the brain-to-body allometry for wasps and bees but is comparable in the number of cells per unit brain mass, indicating a general conservation of brain cell density across these divergent lineages.