Biochemical Mechanisms: Overview Of The Caterpillar Organs
Labial Salivary Glands
As insect attack the leaf by chewing which can be sensed by plants with the biochemical mechanism and we observe gene expression related to plant defense. Like plants, insects also elicit several counter mechanisms. We have seen that plants and the pests have co-evolved for hundreds of millions of years.
The Caterpillars have both salivary glands can be divided into labial and mandibular salivary glands. Both glands although structurally different, provides saliva to establish herbivore on its host plant and can alter plant gene expression. Salivary gland plays an important role affecting host plant chemistry and involving complex ecological interactions The salivary contents of Helicoverpa zea produce not only digestive contents but also several proteins to overcome plant defences. Genomic study of saliva from mandibular and labial glands reveals molecules involvedby mediating in digestion, detoxification, immunity, defense against plant secondary chemicals possibly with the help of other biotic or abiotic factors such as larval stage, temperature, water stress. The salivary contents of Helicoverpa zea produce not only digestive contents but also Glucose oxidase and ATP hydrolyzing enzyme with multiple roles to overcome plant defenses. Celorio-Mancera et al. (2012) also differentiate between labial and mandibular salivary glands regarding more immune related proteins secretion from mandibular gland yet both secretes chemosensory proteins. In another study Musser et. al. (2006) showed that caterpillar salivary glands produce elements such as glucose oxidase (GOX) that can catalyze the formation of hydrogen peroxide (H2O2) to suppress the induction of nicotine in tobacco.
Midgut
Midgut is an important organ in caterpillar which occupies most of the space of it during larval stage. Unlike salivary glands, midgut peritrophic envelope protect the midgut from oxidative damage originated from abrasive food particles, toxins or pathogens. Foregut is the site of temporary storage. On the other hand, water is reabsorbed and fecal forms in the hindgut. Thus, midgut is the most important site in comparison to foregut and hindgut as the latter two sites are cuticularized and lack a peritrophic matrix. On the other hand, changes in midgut endopeptidase activity of caterpillars are responsible for adaptation to plant proteinase inhibitors, which is an anti-herbivore element. According to Heiko Vogel and Richard Musser (2014) the gut is not a playground for detoxification in a caterpillar while malpighian tubules and fat body does. Malpighian tubules are closely associated with the gutIt shows versatile functions and gene expression extensively altered due to adaptation to different feeding habitat. Transcriptional plasticity of the midgut directs polyphagous nature of Anoplophora glabripennis while feeing on diet and expression levels of multicopy genes linked to digestive and detoxification. Unlike many insects such as aphids, honey bees, and termites, caterpillars lack gut microbiome except leaf-derived microorganisms suggesting its typical simple structured gut not suitable for microbial colonization. Caterpillars also do not have gastric caeca and most nutrients are absorbed in the midgut. The midgut tissues also have another importance as Grant et al. (2006) showed the predilection to defensive regurgitate nature of caterpillar which is highly linked with midgut structure.
Fat body
The fat body consists of thin ribbon like sheets of small nodule structured connective tissue and acts as an intermediary organ. Most hemolymph proteins are synthesized here and function as storage of proteins, lipid and carbohydrate. Kilby (1963) compared the fat body to the mammalian liver as it affects homeostatic regulation of the insect blood composition. Storage proteins mostly hexameric glycoproteins are selectively taken up by fat body tissue and utilized later to provide the amino acids necessary for the formation of adult structure. Cheng et. al. in 2006 analyzed gene expression derived from larval and pupal fat bodies from cDNA libraries during metamorphosis stage of silk worm and most were found to be involved in metabolism and cell defense. Furthermore, Wu et al. (2016) demonstrated that Lepidoptera gut immune response depends on the function of the fat body.
Malpighian tubules
Malpighian tubules are closely associated with the gut originates at the pylorus and divides into six tubules with parallel progression 1/2 to 2/3 distance of the midgut. In addition to ion and water transport, malpighian tubules help osmoregulates a broad range of organic solutes and xenobiotics. The Malpighian tubules attribute a classical immune response via the cardinal PGRP/imd pathway along with the detoxification role.
