## 5 Recognition of Biologically Active Compounds
### Sensor-Based Detection of Semichemicals  
In addition to instrumental techniques and behavioral assays, sensorics has also contributed to the development of different tools for the detection of bioactive semiochemicals on insects by means of chemo, bio and nanosensors selective toward semiochemicals. A recent review published by [PERSON] et al., (2018) described the features and important details for the correct uses and scope of sensor-based detection of semiochemicals [185]. In the case of chemosensors, it is mentioned that electrochemical [e.g., potentiometric, conducitmetric, or amperometric], piezoelectric [e.g., quartz crystal microbalance (QCM) and surface acoustic wave (SAW)], and optical [e.g., surface plasmon resonance (SPR)] sensors can be used for semiochemical detection. However, the sensitivity, selectivity, and response time are not comparable to the olfactory systems of living organisms [185; 186]. From this fact, the evaluation of molecular structures involved in odor detection [i.e., hydrophobic proteins known as odorant-binding proteins (OBPs)] is the most common practice to discover/develop biosensors for specific/selective detection of semiochemicals [187]. Due to the significant evolution on the molecular interpretation of insect olfaction and chemoreception [187], OBPs are considered as the central molecular targets in a recently coined approach called \"reverse chemical ecology\" [188], constituting another convenient, reliable and efficient procedure for screening volatile semiochemical candidates [189]. As shown in Figure 1, this approach starts with the identification and expression of OBPs genes from target insects usingbioinformatics and molecular biology-based tools. In this regard, the OBPs genes can be largely selected by RNA sequencing (RNA-Seq) when they are considerably enriched in the antennae of target insects compared with other organs (e.g., legs). This identification is often validated by analyzing the expression of identified genes through quantitative polymerase chain reaction (qPCR) [20]. Direct molecular cloning is the most commonly used approach for OBPs identification, comprising currently three steps such as the design of degenerate primers from conserved protein sequences, subsequent fragment amplification, and full-length sequence assembling through rapid amplification of cDNA ends (RACE) [190]. OBPs gene discovery is also possible using bioinformatics owing to the genome sequencing of various insects. Therefore, another strategy can be performed through a sequence comparison of the target insect genome or expressed sequence tags (ESTs) with available counterparts from other insects looking for OBPs-like orthologs [191]. Once the OBPs are identified in the target insect, such biomolecules are obtained from the original insect or recombinant organisms using protein extraction and purification protocols. Classical molecular methods, such as electrophoresis and sequencing, have also been used for OBPs characterization. The resulting OBP-containing protein extract (or purified OBP) is employed in OBP:ligand-binding and kinetic in-vitro assays for screening the test semiochemicals (previously identified from libraries or in house collections and precisely selected by in-silico methods, e.g., molecular docking [192]). Within the current investigations of OBPs, a few examples can be mentioned such as the measurement of ligand-receptor affinity for OBPs from the moth _Lymantria dispar_ at different pH values [193] and the effect of free fatty acids on the active site proximities [194]; the recognition of an alarm pheromone in an aphid [168]; the assessment of the differences in an OBP in individuals of both sexes in two species of fruit flies by means of proteomics [195]; the identification of six OBPs of the dipteran _Carpomya vesuviana_ and the measurement of their response to semiochemicals [80]; and the characterization of various OBPs of the moth _Plutella xylostella_ present in different organs [196].