## 4 Discussion
### Possible Cross-Talk with Developmental Peptide Hormones? Considering the important role of the fat body in metabolic, developmental and reproductive regulation, we have investigated the levels of _Scg-IRP_, _Scg-NP3_ and _Scg-NP4_, three important peptide hormone encoding transcripts that are known to be expressed in the fat body and play a role in female reproductive physiology [51; 81]. When compared to _dsGFP_ injected control males, the _dsScg-Met_ injected ones had significantly reduced _Scg-IRP_ levels in the fat body (Figure 1C). In many metazoans, the insulin/IGF signaling pathway (ISP) is an important sensor of the nutritional and metabolic status, as well as a regulator of anabolic processes that result in growth and/or reproduction [48; 57]. In _S. gregaria_, only one insulin-related peptide (IRP) (_Scg_-IRP) with main expression in brain and fat body was identified ([56]. Previous studies revealed that _Scg_-IRP expression is regulated during the reproductive cycle and there is evidence for a stimulatory role of _Scg_-IRP on vitellogenesis and ovarian development in adult female desert locusts [51]. In several other insect species, complex functional interactions have been observed between the ISP and JH signaling [7; 14; 48; 82]. In the fruit fly, _D. melanogaster_, the expression of the insulin receptor (IR) was detected in the CA, indicating the ability of the CA to respond to changing insulin-like peptide levels. Moreover, _D. melanogaster_ IR mutants showed significantly reduced JH levels [83]. In the mosquito _Culex pipiens_, the administration of JH rescued the defects observed upon silencing of the ISP [84]. A microarray analysis to explore gene expression in response to the administration of a JH analog in the silkworm, _Bonhyx mori_, demonstrated a stimulatory effect on key regulatory genes involved in the ISP [85]. In the red flour beetle, _T. castaneum_, JH signaling was shown to stimulate the expression of two insulin-like peptide encoding genes [86]. Moreover, in this species, silencing of the JH receptor Met resulted in decreased insulin-like peptide transcript levels [87]. Therefore, our data of the current study extend these widely observed links between JH signaling and ISP towards reproductive development of adult male desert locusts. In this process, an important role seems to be played by the fat body, where the expression of _Scg-Met_, _Scg-Tai_ and _Scg-Krh1_ significantly increased during male sexual maturation (Figure S7), and where in _dsScg-Met_ injected males significantly reduced transcript levels were observed for both _Scg-Met_ and _Scg-IRP_ (Figure 1). In addition, the fat body appeared to be more pronounced in both _Scg-Met_ and _Scg-Tai_ knockdown males than in the _dsGFP_ injected (control) ones (Figure S8). Interestingly, fat body hypertrophy is generally observed in insects with reduced JH and ISP signaling, and in several species this is a naturally occurring phenomenon when the animal is going into a reproductive diapause [48]. This happens in situations when the environmental conditions are adverse, and energy and nutrients are rather saved for the individual insect's survival than invested in gonad development and reproduction. Moreover, it has been shown that a reproductive diapause can be artificially induced in the mosquito _C. pipiens_ by knocking down the insulin receptor via RNAi [84]. Since there appears to exist a functional relationship between nutrient availability, ISP, JH signaling and reproductive maturation,an interesting question could be whether Met knockdown might also affect food intake. There are some studies reporting on a possible involvement of JH in the regulation of digestion by controlling the expression of digestive serine proteases in response to food uptake [88; 89; 90; 91; 92]. In 2007, [PERSON] et al.