Do invertebrates feel pain?

Everyday a huge number of invertebrates are massively killed by pesticides, destruction of habitat, agricultural and industrial practices, cars, others human-related activities and also by natural enemies. However, they remain one of the most abundant groups of organisms on this planet.
Each year 12 million animals are used in research laboratories across Europe: martyrs required for human scientific progress or innocent victims sacrificed for no reasons? The European legislation (Directive 2010/63/EU) has renewed the scientific and ethical conflicts that always accompany animal experimentation. But what are the actual dimensions of the phenomenon? It is true that the current medical and scientific progress would not have been possible without animal research? And above all, nowadays is still need to experiment on animals or are there alternatives? Regarding ethical problems related to animal research, the continuous debate moves broadly over two distinct aspects. In first instance, it is imperative to consider whether animal research yields useful knowledge that cannot be obtained in alternative ways and whether it is morally acceptable to use animals with the possibility to harm them and make them suffer. Vertebrates (and in particular Mammals) more easily trigger compassion feelings and a large number of people and organizations are currently trying their best to protect them. But the invertebrates are largely left to their destiny.
In which degree should we apply to invertebrates the same rights we recognize to vertebrates? Do invertebrates suffer? Are invertebrates capable of feeling pain?
The above questions came to my mind after I read an article reported by a well-known Italian daily newspaper a few days ago, basically it states: Important discovery, when lobsters are dipped in boiling water still alive, they suffer, as do crabs and other crustaceans. The journalist commented on a scientific research, published in the Journal of Experimental Biology [1] that contradicts what was previously thought or widely accepted by people in search of a self-justification (lobster reactions in boiling water are simply automatic nerve reflexes). In fact, researchers have observed the behavior of common crabs subjected to an electric shock. The experiment was designed to clearly distinguish the reactions due to pain from those generated by a reflex movement called nociception. This is a response generated by peripheral nerve endings. While the first is a conscious reaction, the second is a kind of automatism. Until recently it was believed that the crustaceans could not feel pain (but the argument is still debated); this experiment has shown that, after trying the first electric shock, they were trying to hide and to avoid a second shock. Other experiments also support this possibility. I will just mention that opioid peptides and other mammalian-type neurotransmitters (serotonin, dopamine, and acetylcholine, substance P) and opioid receptors were found in crustaceans, and in other invertebrates and since these chemicals mediate pain in vertebrates, their presence in invertebrates could be interpreted as an indication that they may be able to experience pain. However, contrary to what occurs in mammals, for the majority of invertebrates is not adopted any form of protection in the belief that they probably cannot feel pain.
Insects have a nervous system quite different compared to vertebrates. In particular, they present a ganglionic nervous structure characterized by local groups of neurons, called ganglia that are associated with, and specialized for, different body segments. Insects are capable of primitive learning and do exhibit what many scientists would characterize as some form of intelligence. Under an anthropological-based point of view, social insects are endowed with even higher mental capacity. They are able to construct complex societies, acquire, remember and transmit information to their conspecifics. The learning, however, tends to be highly specialized and applicable to only limited domains. In addition to a primitive mental life as described above, there is some evidence that insects can experience pain and suffering. Anyway, insects lack the neurological structures, typical of vertebrates, and in particular mammals that translate a negative stimulus into an emotional experience, suggesting they are not able to process physical stimuli emotionally. Besides, mammals and other vertebrates learn from pain, and change behavior to avoid it in the future. Therefore, pain has a fundamental evolutionary role. Insect behavior, in contrast, is largely a function of genetics. Insects are pre-programmed to behave in certain ways. The insect lifespan is short, so the benefits of an individual learning from pain experiences are minimized. Other behavioral observations also suggest that insects, at least, experience pain in a different way (if they do). In fact, insects with crushed abdomens continue to feed. Caterpillars still eat and move on the host plant, with parasitoids larvae consuming their bodies from inside. A praying mantis male will behave normally and will continue to copulate with female, while she is eating up his head, until the moment of death (gangli controlling mating movements are located in the torax and probably abdomen). Personally I arrived at the conclusion that we should take into consideration the possibility of insect and other invertebrates experiencing pain (maybe with different pathways compared to vertebrates), especially until more evidence will be available. Anyway, the debate on if and how invertebrates feel pain is still open.
For the scientific community, whether pain exists in all animals, but especially in invertebrates, remains a mystery, but surprisingly for some people few invertebrate species seems to deserve special care compared to other phylogenetically related species. With a view opposite to that presented in the previous paragraph, some scientists (I will report just one example) believe that worms twisting on a fish hook feel no pain - nor do crayfish and crabs cooked in boiling water (also supported by a 2005 scientific study funded by the Norwegian government). "The common earthworm has a very simple nervous system - it can be cut in two and continue with its business," Professor W.F. (the name is not important, benefit of the doubt because the sentences are decontextualized) said, and continue "It seems to be only reflex curling when put on the hook ... They might sense something, but it is not painful and does not compromise their well-being" [2]. I could accept this statement if this result is what the experiments indicate but I was surprised to read that W.F. said most invertebrates, including lobsters and crabs boiled alive, do not feel pain because, unlike mammals, they do not have a big brain to read such signals. I am not aware of what kind of experiments draw to these conclusions but I was also surprised to read few lines later: “Some more advanced kinds of insects, such as honeybees which display social behavior and a capacity to learn and co-operate, deserve special care.” It is impossible not this think about the apparent arbitrary of this conclusion. What about bark beetles then? They show sub-sociality and other rather complex behavior (e.g. fungus farming, cooperative breeding and rearing of offspring) Where should we put them on the stair of sorrow? Maybe somewhere in between lobsters and honeybees? Invertebrates cover a range of creatures from insects and spiders to molluscs and crustaceans. In some species of invertebrates the cephalic ganglia are large and complex enough to support some complex behavior (e.g., the lobster and octopus); but if we assume some of these invertebrates actually can feel pain, how is it possible to establish how the evolutionary pathways enabling an organism to experiment these sensations developed in closely related species? What the exhibitions of complex behavior tell us about the way a species experiment the surrounding world? I recognize it is not easy to find an answer to these questions, but these considerations must be taken into account before drawing an arbitrary line assuming, for example that decapods experience pain in different way than isopods (two different orders of crustaceans). To my knowledge, there are no clues on marked differences on the way in which different species, closely related, could experience pain in different ways, but if we cook a lobster in boiling water we normally feel guiltier than if we crush a woodlouse (isopod - the only terrestrial crustacean) under our shoe. Let’s consider the evolution of sight in animals and their variable visual capacity, including the range of wavelength of light they can detect and hence the capacity to discriminate colors, the sensitivity in low light levels and in darkness, the ability to detect fast moving object or resolve objects in high resolution. The eye structure, from the simplest one able to distinguish light form darkness can reach high level of complexity and diversity in taxonomic groups following diverse evolutionary pathways. It is possible to establish what wavelengths an animal eye is able to detect based on DNA sequences of opsin genes, and appreciate the high variation in closely related species or establish the complexity of an eye structure on morphological basis. Maybe we could say that an eagle eye sees better than a lobster eye; more difficult to say how these animals experience the surrounding environment through their eyes produced by such different evolutionary pressures and environments. In the same way, the dimension of the brain or a complex mating strategy, in an invertebrate species, does not provide comprehensive information on the way in which the animal experience pain. In conclusion, the difficulty of drawing a line between the species that can be used without any concern and the pain-sensitive species in the same group, does not by itself justify drawing an arbitrary line.
I wrote it quite fast the reader should shut an eye on a few mistakes on biological/ethical topics treated in a simplified way
Bibliography

1. Magee, B. and Elwood, R. W. (2013). Shock avoidance by discrimination learning in the shore crab (Carcinus maenas) is consistent with a key criterion for pain. J. Exp. Biol. 216, 353-358.
2. http://www.iol.co.za/scitech/technology/worms-and-crayfish-feel-no-pain-experts-1.233383